Home / Statistical Tools / Control Charts / Levey-Jennings / Options
Options¶
Want to build one first?
How-To works through a finished chart step by step, from sample data you copy off the page straight into Excel. The data has a point that is out of control, so the result has something to read rather than twenty points in a row.
How-To with GroupBy does the same thing with a GroupBy column, so one run produces a separate chart per group.
This page covers what is specific to the Levey-Jennings Chart dialog. The parts shared by every control chart, the Data Selection window, and the Data tab, are described once on Create Control Charts. The pane that opens beside the finished chart has its own page, Task Pane. What each option means statistically is on the Math Details page.
The Dialog¶
Levey-Jennings has a dialog of its own. It is not a variant of another chart, so there is no Chart: row of radio buttons above the tabs and no panel to switch on or off: the dialog always builds the one panel, and the title stays Levey-Jennings Chart.
It has three tabs, not four: Data, Limits & Estimation, and Rules & Display. There is no Capability tab. That is not only a hidden tab. The engine also forces the capability index and the histogram off for this chart type, and refuses a capability request that reaches it from anywhere else with "Capability analysis is not available for the Levey-Jennings chart." No Levey-Jennings worksheet ever carries a capability block or a histogram.
Two consequences follow from the missing tab, and both remove interactions that other charts have. There is no specification limit to switch First N subgroups off, and there is no specification-limit-plus-transformation combination to switch Split Control Limits off.
Data Tab¶
The slot lists are Measurement Columns and X-Axis Labels (Optional), alongside the shared Data Types list, the Excel and GroupBy calculation type radios, and the GroupBy region. See Create Control Charts for what each one does.
Two things are specific to Levey-Jennings:
- There is no Subgroups group. This is an individuals-form chart: one row is one plotted point, and there is no subgroup size box, no subgroup identifier drop-down, and no across-columns option. Nothing in the dialog groups your rows.
- The data must be shaped as individuals. If the selection reaches the engine shaped as subgroups, the analysis stops with a message instead of producing a chart: "The Levey-Jennings chart requires a single column of data.", or "The chart type does not match the data shape."
Limits & Estimation Tab¶
This tab carries four groups: Historical Values on the left, and Control Limits, Estimate Limits From, and Split Control Limits on the right.
Two groups you will find on every other variable chart are absent here
There is no Estimate Sigma group. Not a greyed-out one, not one with a single option: the group is not in this dialog at all, and neither are the two unbiased-constants checkboxes. The standard deviation behind the limits is always the overall standard deviation of the data, in its uncorrected form, and the engine rejects a request for unbiased constants with "Unbiased constants are not available for the Levey-Jennings chart." There is also no Transformation group; see Transformation below.
Control limits¶
The Control Limits group on the Limits & Estimation tab sets how the limits on every panel are obtained.
| Control | Type | Default | What it does |
|---|---|---|---|
| Shewhart | radio button | selected | The center line and the limits are computed from the data. |
| Manual | radio button | not selected | You type the limits yourself, one row per panel. |
| None | radio button | not selected | No limits are drawn. The points, the center line, the process statistics, and the outlier markers are all still produced. |
Manual is not offered on every chart and can be switched off by settings on other tabs. The availability table below covers both.
Choosing None also turns the out-of-control tests off. On a variable chart, blanking the limits blanks every test result as well, so no point is flagged, including by the tests that need only the center line. The zone lines checkbox is not greyed out when you choose None, but no lines are drawn, because there is no band left to divide. The finished chart is then written with no zone values at all, so the Analysis Pane hides its Show zone lines checkbox too.
Validation: none of the three radios can produce an invalid state, so there is no message.
Math: Control Limits and Zones.
Number of standard deviations (k)¶
| Control | Type | Default | Accepts |
|---|---|---|---|
| Number of standard deviations (k): | text box | 3 | any number greater than zero, with no upper bound |
This is how far the limits sit from the center line, measured in estimated standard deviations. Lower it and the chart flags more points; raise it and it flags fewer.
Editing k also changes the first out-of-control test. That test asks whether a point lies beyond k standard deviations from the center line, and it reads this same box. That is why it is the one test with no parameter of its own: k is its parameter.
It decides whether there are zone lines at all. Zone lines are drawn only at \(k = 3\), where thirds of the band are one and two estimated standard deviations exactly. Set k to anything else and no zone lines are drawn on any panel, the Show zone lines check box greys with a notice saying why, and the four tests that read the zones do not run.
Where it is hidden. CUSUM has no k box. Its decision interval takes the place of a sigma multiplier.
Validation: blank, non-numeric, zero, and negative entries are all refused, with a message box:
The number of standard deviations must be a number greater than 0.
The box then shows the value it held before. Earlier versions discarded the entry in silence, so it was easy to believe a number had been accepted when it had not.
Math: Control Limits and Zones.
On Levey-Jennings the multiplier is applied to the overall standard deviation, so k is what turns that one number into the drawn band. It is also read by three of the six Westgard rules, which is covered under Rules & Display below.
Manual limits¶
Selecting Manual enables a grid with one row per panel and the column headings Panel, UCL, Center, and LCL. Each cell is a text box and all of them start blank.
Each row is judged on its own, and what you leave blank matters as much as what you fill in.
| What a row contains | What that panel does |
|---|---|
| All three values, with LCL below Center and Center below UCL | Your band is drawn in place of the computed one. |
| Nothing at all | That panel falls back to computed limits, even though Manual is selected. This is deliberate: it lets you set a location panel by hand and leave the dispersion panel to the software. |
| One or two of the three | The chart does not redraw and Finish switches off until the row is completed or cleared. |
| All three, but not in increasing order | The out-of-order entry is refused as you type it, so the row cannot reach the run in that state. |
The three boxes are checked against each other as you fill them in, with a message. Type a UCL at or below the Center or the LCL already in that row, or a Center at or above the UCL or at or below the LCL, or an LCL at or above either of the others, and the entry is refused the moment you leave the box. The message names the panel, so you can tell which row it came from:
The manual UCL for the Individuals panel must be greater than the manual center.
The box goes back to what it held before. A box is only compared against a box that holds a value, so an empty neighbour never causes a refusal and you can fill a row in any order.
A manual limits row needs all three of its boxes, and a half-filled row stops the redraw. Fill one or two of UCL, Center and LCL for a panel, leave the third empty, and the chart does not redraw. The reason appears beside the Finish button and Finish switches off:
Every panel with a manual limit needs all three of UCL, Center and LCL.
No message box opens for this one either, for the same reason as an empty First N count: a row is filled one box at a time, and a message box after the first entry would interrupt an entry still in progress. Nothing you typed is removed. Fill the third box, or clear the row, and the chart redraws.
A row left completely empty is not a half-filled row and is fine: the panel simply takes its calculated limits.
Values you type survive a switch away from Manual, but they do not act. Choosing Shewhart or None leaves the grid as you left it and stops it from affecting the run. Switching back to Manual makes those values live again.
Zones and tests follow the band you typed. On a panel that carries zone lines, and with the sigma multiplier still at 3, the lines are placed at thirds of your band, computed separately above and below the center, so an asymmetric band gets asymmetric zones. The first out-of-control test means "outside the typed band" rather than any particular multiple of a sigma estimate.
The process statistics are still computed from the data. The mean and the sigma estimates continue to be estimated and continue to be reported in the summary table, even though they no longer draw the band.
Math: Control Limits and Zones.
On Levey-Jennings the grid has one row, named Levey-Jennings. There is no second panel, so the "fill one row and leave the other blank" pattern that the subgrouped charts use does not arise here: either the row is complete and your band is drawn, or it is empty and the band is computed, or it is part-filled and the panel is replaced by a red message.
Historical (known) values¶
The Historical Values group lets you supply a number instead of estimating it, so the limits describe a process you have already characterized rather than the data in front of you. Every box is a text box, every box starts blank, and blank means estimate it from the data.
| Control | Type | Default | What it replaces |
|---|---|---|---|
| Mean: | text box | blank | the mean behind the center line |
| Standard deviation (overall): | text box | blank | the overall sigma |
| Standard deviation (within): | text box | blank | the within-subgroup sigma |
| Mean (between): | text box | blank | the mean of the subgroup means |
| Standard deviation (between): | text box | blank | the between-subgroup sigma |
Which boxes appear depends on the chart, because a box for a quantity the chart never uses would be misleading.
| Chart | Boxes shown |
|---|---|
| Xbar-R, Xbar-S, I-MR | Mean, overall, within |
| I-MR-R/S | all five |
| EWMA, CUSUM | Mean, within |
| Levey-Jennings | Mean, overall |
Behavior worth knowing:
- A supplied value wins, and it is applied last. The estimate is computed from the data first and then overwritten, so a First N baseline still selects which points fed the discarded estimate, and still governs anything you did not supply.
- One number can change two. Supplying a within sigma also redefines the between sigma, and supplying a between sigma redefines the sigma of the subgroup means, because the remaining quantity is back-solved from the data.
- A supplied sigma carries no degrees of freedom, so it gets no confidence interval in the capability output. A supplied mean affects no degrees of freedom.
- One value covers every phase. With split control limits on, the same supplied number applies to all phases; there is no per-phase historical value.
- A transformation discards them for that run. Turning on Box-Cox or Johnson greys out the whole group and the values are not used. What you typed reappears if you turn the transformation off.
Validation: every one of these boxes is now checked in the dialog, and refuses with a message box naming the box. The three standard deviation boxes take a number greater than zero, or blank:
The historical standard deviation (within) must be a number greater than 0, or blank.
The two mean boxes take any number, or blank, because a mean may legitimately be negative:
The historical mean must be a number, or blank.
In both cases the box shows the value it held before. Earlier versions range-checked none of these boxes, so an unusable value passed the dialog and showed up as an unusable chart.
Math: Process Sigma Estimation.
Levey-Jennings shows two of those boxes, Mean: and Standard deviation (overall):. The within and between boxes are not shown, because the chart never forms a within-subgroup or a between-subgroup estimate, and the engine refuses either override with "The Levey-Jennings chart uses overall sigma only; within- or between-sigma overrides are not available." A supplied overall standard deviation must be greater than zero; anything else stops the run with "The historical sigma override must be greater than zero."
A historical mean together with manual limits¶
These two are not mutually exclusive on most charts, and the result differs by chart. None of the three behaviors is announced, so read the row for your chart before combining them.
| Charts | What happens |
|---|---|
| Xbar-R, Xbar-S, I-MR, I-MR-R/S | Both settings act, on different parts of the output. The chart draws the center line you typed in the manual grid, while the summary table reports the historical mean. The two disagree by design, so expect it rather than treating it as an error. |
| EWMA, CUSUM | The combination cannot arise. Manual limits are not offered on either chart, so a historical value is the only one of the two you can set. |
| Levey-Jennings | The combination is blocked in both directions. Entering any historical value greys out Manual, and selecting Manual greys out the whole Historical Values group. |
| p, np, Laney P', c, u, Laney U' | The historical box disappears when you select Manual and its value is ignored for that run, because nothing is estimated under manual limits. Switching back to Shewhart brings both the box and its effect back. |
Manual limits and historical values are mutually exclusive¶
This gate exists on this chart alone, it works in both directions, and it acts as soon as you type or select:
- Type a value into Mean: or Standard deviation (overall): and the Manual radio in the Control Limits group is disabled. If Manual was already selected, the selection reverts to Shewhart.
- Select Manual and the whole Historical Values group is disabled.
Clear the box, or move off Manual, and the other side comes back. Nothing you typed is lost either way. The reasoning is that both settings are ways of placing the same two lines, so accepting both would leave the chart with two answers; the engine refuses the combination as well, with "Historical mean/sigma overrides cannot be combined with manual control limits."
What switches these options off¶
Four combinations of settings make an option on this tab unavailable. Every one of them acts the moment you type or select, and none of them shows a message, so it is worth knowing them before you go looking for a control that has greyed out.
| When this is set | This becomes unavailable |
|---|---|
| A Box-Cox or Johnson transformation | Manual limits. The radio greys out, and a Manual selection reverts to Shewhart. The Historical Values boxes are also ignored for that run. |
| Any split mode other than None, whether or not you have entered break points yet | Manual limits, in the same way. Selecting the mode is enough; the gate does not check whether the split produces anything. |
| A lower or upper specification limit on the Capability tab. A target alone does not do this. | First N subgroups under Estimate Limits From. The selection reverts to All subgroups. Levey-Jennings, EWMA, CUSUM and Z-MR have no Capability tab, so this gate cannot arise on them. |
| A transformation and a specification limit together | Split control limits. The whole Split Control Limits group greys out and the mode reverts to None. |
The specification-limit gate reads the two specification boxes only, so a Target: on its own leaves First N subgroups: selectable. That combination does not produce a chart: a target alone is enough to turn the capability calculation on, and the engine refuses to combine capability with a First N baseline, so the run stops with "Capability cannot be combined with a First-N baseline." Clear the target, or return Estimate Limits From to All subgroups.
Two chart-level rules sit alongside them: EWMA and CUSUM never offer manual limits at all, and Levey-Jennings treats manual limits and historical values as mutually exclusive, so entering either one greys out the other.
What Estimate Limits From (Baseline Estimation) does¶
It changes what the limits are estimated from. It never changes what is plotted. Every point always appears on the chart. Asking for the first 25 subgroups does not hide subgroup 26 onward. It means the center line and the control limits are computed from the first 25 subgroups and then drawn straight across the whole chart, so the later points are judged against the earlier baseline. That is the entire purpose of the setting: it is how you freeze a known-good period and watch what happens afterwards.
The same setting carries two labels. It is Estimate Limits From on the variable charts and Baseline Estimation on the attribute and rare-event charts, because those dialogs were written separately. The intent is identical, but the smallest count you may enter differs, and so do a few of the edge cases, so use the block written for this chart.
| Estimated from the selected points only | Computed from all the points regardless |
|---|---|
| The mean behind the center line | Every plotted value |
| The overall sigma | Each subgroup's own mean, range, and standard deviation |
| The within-subgroup sigma and everything it is built from | The minimum, the maximum, and the count |
| The between-subgroup sigma | The observed parts per million outside the specification |
| The moving-range estimates on an individuals chart | The histogram and the distribution fit |
| The estimated proportion or rate on an attribute chart | The out-of-control tests, which are applied to every point |
| The reported degrees of freedom | |
| The transformation itself |
Two of those rows surprise people:
- The transformation is fitted on the baseline too. A Box-Cox lambda or a set of Johnson parameters is fitted from the baseline points only and then applied to every row in the data. Change the baseline and you change the transformation, not just the limits.
- The degrees of freedom come from the baseline, so a short baseline widens the capability confidence intervals as well as changing the limits.
With split control limits on there is still only one count. It is reused inside every phase, capped at the length of that phase, so a phase shorter than the count simply uses all of its own points. The count never reaches across a phase boundary, and there is no way to give one phase a different baseline from another.
Math: Baseline and Limit Estimation.
Estimate Limits From¶
| Control | Type | Default | Accepts |
|---|---|---|---|
| All subgroups | radio button | selected | |
| First N subgroups: | radio button | not selected | |
| the count box beside it | text box | blank | a whole number of 5 or more. Its tooltip reads "Minimum 5 subgroups". |
The count is in plotted points, not in worksheet rows. On an individuals chart it counts points, and the moving ranges available inside that window are one fewer than the count at the default moving range length, fewer still if there are gaps.
| What you do | What happens |
|---|---|
| Select First N subgroups: and leave the box blank | The chart does not redraw and Finish switches off, with the reason beside it. No message box opens. See below. |
| Type a number below 5 | Refused with a message box reading "The first N subgroups must be a whole number of 5 or more.", and the box goes back to what it held before. |
| Type something that is not a number | Refused with "The first N subgroups must be a number.", and the box goes back to what it held before. |
| Type a number larger than the data | Every point is used. |
| Leave a blank subgroup inside the window | It does not use up one of the N. The window reaches further into the data to find real subgroups. |
| Exclude a point as an outlier inside the window | The same: it does not use up one of the N. |
The minimum of 5 is the variable chart engine's own: it refuses to compute limits from fewer than five points. The attribute and rare-event charts use a minimum of 4, and the T-squared and Generalized Variance chart uses 2.
Selecting First N subgroups with the box beside it empty stops the redraw, and says so quietly. Choose the First N subgroups radio before typing a count and the chart does not redraw, the reason appears beside the Finish button, and Finish switches off:
First N subgroups is selected, so the box beside it needs a number.
No message box opens for this one. You have selected the radio and have not yet had a chance to type, so the dialog waits rather than interrupting. Type a count and everything continues. A box you have not finished filling in is the one kind of refusal that stays quiet; a rule about two settings that disagree with each other still opens a message box.
The box is not filled in for you either. A count equal to every subgroup would mean the same thing as All subgroups, and it would write a number into the worksheet's settings note that nobody typed.
Before this, selecting the radio with an empty box redrew the chart from all subgroups while the settings note read "First 0 subgroups", so the note stated a baseline the chart had not used. A worksheet saved before this rule existed can still hold that state, and the settings note now prints All subgroups for a count of 0 rather than "First 0 subgroups".
A specification limit switches this off. Type a lower or an upper specification limit on the Capability tab and First N subgroups: greys out and the selection reverts to All subgroups, because a frozen baseline and a capability calculation disagree about which data describes the process. Levey-Jennings, EWMA, CUSUM and Z-MR have no Capability tab, so this cannot arise on them. There is no message, and clearing the specification limit makes the radio available again.
A target on its own does not switch it off, and the combination then fails
The dialog gate reads the LSL: and USL: boxes only, so a Target: with both specification boxes empty leaves First N subgroups: selectable. It should not be. A target on its own is enough to turn the capability calculation on, and the engine refuses to combine any capability calculation with a First N baseline, so pressing Finish in that state produces no chart at all and the message "Capability cannot be combined with a First-N baseline."
To avoid it, either clear the Target: box or set Estimate Limits From back to All subgroups.
Every phase still needs at least five usable points. With split control limits on, one phase too short fails the whole analysis with a message instead of a chart, whatever this setting says.
Validation: the count box refuses a number it cannot use, with a message box, and keeps what it held before. Earlier versions corrected the count in place and ran the analysis without reporting anything.
On Levey-Jennings the count is a count of plotted points, which on this chart is a count of usable measurement rows, so First N subgroups: 25 estimates the mean and the overall standard deviation from the first 25 usable values. The specification-limit gate described in that block cannot be reached here, because this chart has no Capability tab.
Transformation¶
Transformation¶
A transformation replaces each measurement with a rescaled version of itself, chosen so the rescaled values sit closer to a normal distribution. Control charts assume a normal distribution and never fit any other one, so a transformation is the only remedy the product offers for skewed measurement data on a control chart.
| Charts | Transformation group box |
|---|---|
| Xbar-R, Xbar-S, I-MR, I-MR-R/S, EWMA, CUSUM | Offered |
| Levey-Jennings, Z-MR | Not offered. There is no control, and no transformation is applied. |
| p, np, Laney P', c, u, Laney U', G, T | Not offered. These charts plot counts, proportions, or waiting times and have their own distributions built in. |
The dialog shows no Transformation group box, and the engine refuses one if a transformation reaches it from a restored analysis or from any other route, with "A Box-Cox or Johnson transform cannot be used with the Levey-Jennings chart." A Levey-Jennings chart is always drawn in the units you measured in.
Split Control Limits¶
A split divides the chart into consecutive sections, called phases, and gives each phase its own center line and its own control limits. Use it when the process changed on a known date: a new supplier, a tooling change, a settings change. The limits then step at the change instead of averaging across it.
Every phase estimates its own center line, its own sigma, its own control limits, and its own zone lines where the chart has them, and the out-of-control run counters restart at every phase boundary. Splitting never hides or moves a point: all points stay on the chart in their original order, and only the limits they are judged against change. For the formulas, and for the full list of what is shared across the chart versus estimated per phase, see Splits and Phases.
Controls, variable chart dialogs¶
The group box is headed Split Control Limits and holds three mutually exclusive mode buttons plus the editors each mode reveals.
| Label as shown | Control | Default | Valid entries | What it does |
|---|---|---|---|---|
| None | Radio button | Selected | n/a | One center line and one set of limits for the whole chart. |
| Split by column | Radio button | Not selected | n/a | A new phase starts at every row where the chosen column's value changes. |
| (split column list) | Drop-down list, no label of its own | Nothing selected | Any one column offered in the list | Names the column whose value changes drive the phases. |
| Manual break points | Radio button | Not selected | n/a | Phases start at the point numbers you type. |
| (break entry box) | Text box, no label of its own. Tool tip: "Enter a 1-based chart point (subgroup) number" | Empty | A whole number from 1 up to the number of plotted points | Holds one break number until you press Add. It is a staging box; typing in it alone changes nothing. |
| Add | Button | n/a | n/a | Moves the typed number into the break list, in ascending order, and inserts a matching label slot. |
| Remove | Button | n/a | n/a | Deletes the break selected in the list and its label slot. |
| (break list) | List box | Empty | n/a | Shows the break point numbers currently in effect. |
| Split Labels (shown beside each divider): | Label above one text box per phase | One empty box, named "Split 1" | Any text | The text drawn beside each divider on the chart. |
Hidden rather than disabled: the split column list appears only while Split by column is selected, and the break entry box, Add, Remove, the break list, and the whole Split Labels block appear only while Manual break points is selected.
Disabled: the entire group box is greyed out, and the mode reverts to None, when a transformation and a capability analysis are both active. See the conditional behavior table below.
Validation. Add refuses an entry it cannot use with a message box, and leaves what you typed in the box so you can correct it. Each way the entry can be wrong has its own sentence:
| What you typed | The message |
|---|---|
| Anything that is not a whole number | The split row must be a whole number. |
| A number below 1 | The split row must be a whole number of 1 or more. |
| A number at or beyond the number of plotted points | The split row must be less than the number of plotted points (120). |
| A break already in the list | The split row must be a row that is not already in the list. |
A split starts a new phase, so it has to fall strictly inside the data: row 1 is the first row that can start a second phase, and the last plotted point cannot start one at all. When the point count is not yet known to the dialog the upper check is skipped, so a break past the end of the data can be added and is dealt with later by the calculation.
Remove with nothing selected in the list does nothing and says nothing. There is still no check on how short a phase the break would create.
Earlier versions refused all four cases in silence, leaving the entry in the box with nothing on screen to say why the list had not grown.
What a break number means¶
A break number is the 1-based number of the plotted point that starts the new phase, the same number you read along the horizontal axis, not a worksheet row address. On charts plotted from subgroups it is the subgroup number, not the raw data row. On charts where one row is one point the two are the same.
Breaks must be different from each other and are kept in ascending order for you. A break at point 1 does nothing, because the first phase already starts there.
Split Labels¶
The Split Labels (shown beside each divider): block gives one text box per phase, so the number of boxes is always the number of breaks plus one. Each box is named Split 1, Split 2, and so on, in chart order. Those names are slot names, not the labels: they are fixed and cannot be edited. Whatever you type in the box beside a slot is what gets drawn on the chart. Leaving a box empty draws no label for that phase.
Adding a break inserts a new empty box at the position that changed rather than adding one at the end, so labels already typed stay with the phases they were written for.
The block appears only in the manual break mode, so there is no way to type phase labels when the phases come from a column. The labels are never read from the split column's values. Labels typed earlier in manual mode are kept when you switch to column mode, and they are still drawn beside the dividers; without them the dividers carry no labels.
The split column is claimed from the Data tab¶
The split column is chosen here, but it is one of the worksheet columns listed on the Data tab, and a column can hold only one job at a time. Choosing a column as the split column claims it: on the Data tab that column is greyed out and can no longer be checked as a measurement column, an X-axis label column, a subgroup ID, or a part column. Releasing it here gives it back. A column already claimed for one of those other jobs appears greyed in the split column list and cannot be picked. Neither the greyed check box on the Data tab nor the greyed list entry here explains which job is holding the column.
Selecting the column mode with no column chosen is not an error. It is a deliberate decision in the product, and it means do not split. Nothing is flagged, no message appears, and the chart is built with one set of limits, exactly as if the mode were None. The same is true of the manual break mode with an empty break list.
Minimum phase length, and two opposite outcomes¶
Every phase has to be long enough to estimate limits from. The minimum differs by chart family, and so does what happens when a break breaks the rule.
| Charts | Shortest phase allowed | A break that would create a shorter phase |
|---|---|---|
| Variable charts | 5 points | The whole analysis is rejected and no chart is produced. |
| Attribute and rare-event charts | 2 points | The break is dropped and the chart is built without it. |
That is the same user action with opposite outcomes: on an attribute chart a too-short phase costs you a divider, on a variable chart it costs you the chart.
On a variable chart the rejection reads The baseline must use at least 5 subgroups., the same message a short baseline gives, even though the cause is a phase that is too short.
The variable chart dialogs validate neither number. They do not check the minimum, and they do not check the arithmetic of the phase lengths your breaks imply, so a set of breaks can be accepted without complaint at entry and then fail when the chart is built. The attribute and rare-event dialogs do check the two-point rule as each break is added, with the message shown above.
A variable chart phase also fails the analysis when it holds fewer than five values that are actually present, even when the phase itself is long enough. Blank rows inside a phase therefore matter as much as its length.
On Levey-Jennings the five-point minimum is five observations, and one exception applies to this chart and to no other. When the limits do not depend on the data at all, the check is waived: that is when both Historical Values boxes are filled, or when manual limits are supplied. In either of those cases a Levey-Jennings chart will plot from fewer than five points, and from data with no spread in it. On the ordinary path, where the mean and the standard deviation are estimated from your data, the minimum applies as it does on every other variable chart, and it is applied inside each phase separately.
Conditional behavior¶
| Condition | Effect |
|---|---|
| Any split mode other than None is selected, on a variable chart | Manual control limits become unavailable. The Manual option is greyed out, and if it was already selected it reverts to the calculated limits. |
| A split mode is selected but produces no phases (column mode with no column, or manual mode with an empty list), on a variable chart | Manual control limits are still unavailable. The check looks at which mode button is selected, not at whether any phase boundary results. Setting the mode back to None restores the Manual option. |
| Any split mode selected, on an attribute or rare-event chart | No such restriction. Manual limits stay available, and the band you enter is drawn in every phase. |
| A transformation and a capability analysis are both active | The whole Split Control Limits group box is greyed out and the mode reverts to None. A capability analysis counts as active as soon as a lower or upper specification limit is entered. This can only arise on the charts that offer both a transformation and a Capability tab. |
| A transformation is active, with or without splits | One transformation is fitted for the whole chart and then applied to every row. Phases never get their own transformation. |
| Estimate Limits From, or Baseline Estimation, is set to a first-N count | There is one count for the whole chart. It is reused inside every phase and capped at that phase's length, so a short phase uses all of its own points while a longer phase in the same chart uses the count you typed. There is no per-phase count, and the count never reaches across a phase boundary. See Baseline and Limit Estimation. |
| Out-of-control rules are on | Every run and every window restarts at each phase boundary. A run in progress when a boundary arrives is abandoned, so a run of eight points that straddles a boundary cannot combine into one signal. The point-by-point test against the limits is unaffected, because each point is compared with its own phase's limits. |
| A historical mean or sigma is supplied | The one value you supply is applied to every phase. It is not a per-phase entry. |
Splits are edited after the fact in the Control Chart task pane as well, but only the manual break points and the labels. The column mode cannot be selected or cleared there.
Rules & Display Tab¶
Out-of-Control Rules¶
The group box headed Out-of-Control Rules holds one row per rule. Each row is a check box carrying the rule's description, followed by up to two small numbered boxes labelled N = and M =. N is how many points the rule needs, and M, where a rule has one, is the length of the window those points must fall inside.
Ticking a rule adds its test to the analysis. Nothing else about the chart changes: the center line, the limits, and the plotted points are the same whether a rule is on or off.
Thresholds. Every box arrives pre-filled with that rule's standard value rather than blank, so you can see what the rule will do before you change it. Clearing a box means use the standard value; it does not mean zero. Unticking a rule leaves its numbers visible but locks the boxes, so the value you set is still there when you tick the rule again.
A threshold the rule cannot use is refused with a message box naming the rule and the box, and the box goes back to the value it held before:
The rule 2 N must be a whole number of 2 or more.
The rules that count consecutive points, rules 2, 3 and 4, need an N of 2 or more: at an N of 1 every point in the chart would be out of control, because every point is on one side of the center line or the other.
The two rules that count N points inside a window of M, rules 5 and 6, are checked against each other as well. N takes a whole number of 1 or more, M a whole number of 2 or more, and an N larger than the M beside it is refused with the other box's value in the message:
The rule 5 N must be a whole number no greater than the rule 5 M (3).
The comparison runs only when both boxes hold a value, so you can fill them in either order. The Westgard Sigma = box takes any number greater than 0.
Earlier versions said nothing when a threshold was refused: a count below 2 was raised to 2 as you left the box on the attribute and rare-event dialogs, and a blank, zero or negative entry was treated as the standard value on the variable chart dialogs.
The rule set on the Levey-Jennings chart¶
This chart replaces the numbered rules with the six Westgard multi-rule tests, in this order, and all six are ticked by default.
| Row label as shown | Parameters and defaults | Default |
|---|---|---|
| 1-2s - one point beyond 2 sigma (warning) | Sigma = 2 | On |
| 1-3s - one point beyond the control limits | none | On |
| 2-2s - N of M consecutive beyond 2 sigma (same side) | N = 2, M = 2 | On |
| R-4s - consecutive pair straddling ±2 sigma | none | On |
| 4-1s - N of M consecutive beyond 1 sigma (same side) | N = 4, M = 4 | On |
| 10x - N consecutive on one side of the mean | N = 10 | On |
R-4s is offered nowhere else in the product. Rules 3, 4, 7, and 8 are not part of the Westgard set and never run on this chart.
The chart also offers the sigma multiplier, and it defaults to 3. Leave it at 3 if you want these rule names to mean what they say: the rules that read the 1 sigma and 2 sigma lines read lines placed at thirds of the drawn band, so at any other multiplier those lines are no longer at 1 and 2 sigma and the names stop matching what is being tested.
All six rules run on the one panel. There is nothing here of the location-panel and companion-panel split that the two-panel and three-panel charts have: the single panel is the location panel, so no rule is greyed out for want of somewhere to run, and the dialog shows no note about which panel a rule applies to.
Moving the sigma multiplier away from 3 stops three of the six running at all. Set Number of Standard Deviations (k): to any other value and 2-2s, R-4s, and 4-1s do not run, whatever their check boxes say, because all three read the zone lines and zone lines exist only at three standard deviations. The dialog greys its Show zone lines box and says so, and the finished worksheet lists only the rules that ran.
The other three keep working at any width, and one of them moves with it. 1-3s moves, because it tests the drawn limits themselves, which sit at \(k\) sigma. 1-2s does not move: it recovers one sigma from the limit spacing rather than reading a zone line, and on this chart that recovery returns the overall standard deviation exactly whatever \(k\) is, so it always fires at a true two sigma. 10x does not move either, because it only counts points on one side of the center line.
The sigma multiplier is also a rule setting
Rule 1 tests each point against the drawn control limits, so editing the number of standard deviations in the Control Limits section changes the Rule 1 test at the same time as it changes the limits. That is why Rule 1 has no threshold box of its own. It also decides whether rules 5 through 8 run at all: they read the zone lines, and zone lines exist only at three standard deviations, so at any other width those four do not run whatever their check boxes say.
Conditional behavior¶
| Condition | Effect |
|---|---|
| Control limits set to None, on a variable chart | No rule fires. Every rule result is cleared along with the limits, so no point is marked even though the rules are still ticked. |
| Control limits set to None, on an attribute or rare-event chart | The center line is still estimated, so rules 2, 3, and 4, and Rule 9 on a G chart, still fire and still mark points. Rule 1 cannot fire, because there are no limits to exceed. |
| Control limits set to Manual | The rules run against the band you entered, and the zone lines are derived from that band, still only on a location panel and still only at a sigma multiplier of 3. |
| Manual limits entered with the center left blank, on an attribute chart | There is no center line, so rules 2, 3, and 4 cannot fire. |
| Splits are in effect | Every run and every window restarts at each phase boundary. A run in progress when a boundary arrives is abandoned and cannot combine with the run that starts after it. |
| A point is blank or excluded | Runs break at the gap rather than bridging it. For the two window rules, rules 5 and 6, a blank invalidates every window that overlaps it, not just the run. |
| A rule is unticked | Its threshold value is kept and its box is locked. Nothing is lost by turning a rule off and on again. |
| The sigma multiplier is moved off 3 | No zone lines are drawn on any panel, and rules 5 through 8 do not run. The dialog greys its Show zone lines box and says so, and the Rules in effect block on the worksheet lists only the rules that ran. Z-MR is fixed at 3 and is never affected. |
| A capability analysis or a transformation is active | Neither affects the rules. The rule set, the thresholds, and the panels checked are the same. |
See also: Out-of-Control tests.
Two rows of that table cannot arise on this chart. There is no transformation to activate and no capability analysis to attach, so the row covering them is inapplicable here. Where the table refers to rules 5 through 8, read it as the two Westgard rules that carry those numbers, 2-2s and 4-1s. Rules 3, 4, 7, and 8 have no Westgard counterpart and never run here.
The chart does not tell you which rule fired
Every rule you tick is tested, and the results are combined into a single verdict per point: this point broke at least one enabled rule, or it did not. Points that broke one are drawn in a different color from the rest, and that is the whole of the report. There is no per-rule label, no per-rule marker, no per-rule column, and no list of violations anywhere in the chart or the worksheet. A point that breaks four rules at once looks exactly like a point that breaks one, and the output never names either.
So when you turn several rules on, read a marked point as an instruction to go and look at the chart, not as a diagnosis. If you need to know which pattern caused a particular signal, the practical method is to switch the other rules off and rebuild the chart.
Display Options¶
The Display Options group box holds two features: the sigma zone lines, and reference lines of your own at Y values you choose. The variable charts take it from a shared control. The p, np, Laney P', c, u, Laney U', G, and T dialogs repeat the same block for the reference lines, but carry no zone line checkbox, because those charts draw no zone lines.
- Show zone lines: draws four light grey lines inside the control band, two above the center line and two below, dividing each side into three zones. On a chart with more than one panel the label names the panel the lines belong to, because only that panel gets them: Show zone lines (Xbar chart only) on Xbar-R and Xbar-S, Show zone lines (Individuals chart only) on I-MR and I-MR-R/S, and Show zone lines (Z chart only) on Z-MR.
Zone lines are drawn on one kind of panel only. They belong to the panel that plots the measurement against a band that is symmetric about the center line: the Xbar panel, the Individuals panel, and the Z panel of a Z-MR chart. The dispersion panels (Range, Standard Deviation, Moving Range) never carry them, and neither do the EWMA, CUSUM, T-squared, and Generalized Variance panels, nor any attribute or rare-event chart.
And only at three standard deviations. Set Number of standard deviations (k): to anything other than 3 and the checkbox is greyed, with a notice beside it reading "Zone lines are shown only when the control limits are at 3 standard deviations." Nothing is drawn at any other width. Z-MR is the exception: its limits are fixed at 3, so its box never greys.
This checkbox decides whether the lines start out visible. Where they exist, they can be shown or hidden later from the Analysis Pane without running the analysis again; where they do not exist, the pane hides its checkbox rather than offering one with nothing to act on.
Several of the out-of-control tests measure a point against these zones, so a chart with no zone lines runs no zone test. Which tests those are is on Out-of-Control tests, and where the lines are placed is on Control Limits and Zones.
Setting Control limits to None leaves the checkbox available, but with no control band there is nothing to divide into zones and no lines are drawn. The chart is then written with no zone values at all, so the Analysis Pane hides its Show zone lines checkbox for that chart too.
Reference lines¶
A reference line is a horizontal line at a Y value you supply: a specification limit, a target, a contract threshold, anything you want the points compared against. Until you add one the editor reads "No reference lines configured. Use the row editor below to add Y-value reference lines."
Each reference line is one row with four editable cells and a remove button:
| Cell | What it sets | Default |
|---|---|---|
| Value | the Y position of the line | 0 |
| Label | the legend entry for the line | blank |
| Color | the line color, picked from a color button | red |
| Thickness | the line weight, chosen from 0.5, 1.0, 1.5, and 2.0 | 1.0 |
| X | removes that row. Its tooltip reads "Remove this reference line" | n/a |
Value is not validated. Type something that is not a number and the cell keeps the value it had. A blank Label keeps the line out of the chart legend; the line is still drawn, under a generated name built from its value. The editor scrolls on its own once the list of rows is taller than the space allowed for it.
- Add Reference Line: appends a blank row and refreshes the chart preview by itself. Removing a row with its X button does the same.
- Apply: commits edits to Value, Label, Color, and Thickness, which do not refresh the preview on their own. Its tooltip reads "Refresh the chart preview to reflect any pending reference-line edits". The button appears only once at least one reference line exists, and does nothing when there is nothing outstanding to commit.
So adding and removing lines is immediate, while editing an existing line needs Apply.
Reference lines are drawn on the panel that plots the measurement itself. On Levey-Jennings that is the only panel there is, so a reference line you add is always drawn, and the Y axis is stretched to include its value even when it falls outside the data.
Layout you do not choose¶
Nothing else about the appearance of the chart is a dialog option. These are fixed:
- Which panels are drawn is not part of Display Options. On Xbar-R, Xbar-S and I-MR it is the Chart: radio group that sits above the tabs, and the ribbon entry you started from only decides which of those buttons is already selected when the dialog opens. You can change it there without closing the dialog. Every other control chart draws a fixed set of panels and offers no control over it.
- Panel titles are composed for you from the panel type, the group, and the measurement column name, with empty parts dropped.
- Plot order is fixed, and it is also the front-to-back order: the data line, then the zone lines on a panel that has them, then the upper and lower control limits, then the center line, then the reference lines. The zone lines are added before the limits deliberately, so they stay in the background.
- Axes are set for you. The X axis is numbered by plotted point and titled Subgroup. The Y axis is scaled to fit every plotted value, including the zone lines on a panel that draws them and, on the location panel, the reference lines; dispersion panels are held at or above zero. When a chart has more than one panel the panels are aligned with each other so their points line up vertically.
Once the chart exists, zone lines that were drawn can be shown or hidden from the Analysis Pane without running anything again. Zone lines that were never drawn cannot be brought back from the pane: the pane hides its checkbox, and the only way to get them is to modify the analysis, set the control limits to 3 standard deviations, and run it again. Reference lines are not in the pane at all, so to add, edit, or remove one you modify the analysis and run it again.
Task Pane¶
The Control Chart pane that opens beside a finished chart has its own page: Task Pane.
When a number box refuses what you typed¶
A refused number box now tells you why. Type something this box cannot use, then leave it, and a message box opens naming the box and what it wanted:
The box name must be what it wanted.
The title is the analysis name, for example "G Chart" or "Type 1 Gage Study". A box that wants a proportion adds a second sentence, "It is a proportion, not a percent, so type 0.05 for 5 percent.", because typing 5 for five percent is the mistake it is there to catch. The box then goes back to the value it held before, so nothing is lost and nothing wrong is carried into the run.
Text that is not a number at all gets the same message. Type a word, a stray letter beside a figure, or nothing but a minus sign into any number box on this dialog, leave the box, and a message box opens reading:
The box name must be a number.
The box then shows the value it held before. Before this, a box given text simply would not take it: the entry was dropped on the way to the setting, so none of the rules below could be reached and nothing was said. The box looked as though it had accepted what was typed until the moment the focus moved.
The dialog no longer runs the analysis while a setting is unusable. Changing almost any option on this dialog redraws the chart straight away, without waiting for Finish. That redraw now reads the same rule the Finish button reads. When something on the dialog is wrong, the redraw does not happen and the dialog stays open, with the reason printed beside the Finish button.
Where the problem is two settings that contradict each other, and you have finished making both choices, a message box says so as well. Where it is a box you have not finished filling in, the dialog waits quietly instead, with the reason still beside Finish.
Nothing you typed or clicked is undone. The setting you just changed stays changed, the number you typed stays in its box, and the same sentence stays on screen beside the Finish button, which is switched off until you resolve it. Change the other half of the pair, or put the number back, and the chart redraws as usual.
A message box is shown once per reason, not once per redraw. A single click can raise a dozen redraws internally, and repeating the same message box a dozen times would be worse than saying it once, so a second attempt that hits the same reason shows no message box. That is not the dialog ignoring you: the reason is still printed beside Finish the whole time.
This matters because these two-setting rules were previously unreachable. The sentence beside Finish was written and Finish was switched off, but the redraw ran the analysis anyway without ever consulting either, so the chart was drawn from the settings the dialog had already refused.
These boxes are shared by every variable control chart dialog, so they refuse in the same words wherever they appear. A dialog that does not offer a control does not show its box at all.
| Box | What it must be |
|---|---|
| Number of standard deviations (k): | a number greater than 0 |
| Mean: and Mean (between): under Historical Values | a number, or blank. A process mean may be negative, so nothing is ruled out except text. |
| Standard deviation (overall):, (within): and (between): under Historical Values | a number greater than 0, or blank |
| Manual UCL, Center and LCL, per panel | a number, or blank, and in increasing order down the row. The message names the panel: "The manual UCL for the Individuals panel must be greater than the manual center." |
| First N subgroups: count | a whole number of 5 or more |
| Moving-range length (L): | a whole number from 2 to 100 |
| The custom Box-Cox lambda box, hinted (-5 to 5) | a number from -5 to 5 |
n = beside Constant size: |
a whole number from 2 to 100, or from 1 to 100 on a chart that accepts individual values |
| N = on the rules that count consecutive points (2, 3 and 4) | a whole number of 2 or more. The message names the rule: "The rule 3 N must be a whole number of 2 or more." |
| N = and M = on the N-of-M rules (5 and 6) | N a whole number of 1 or more, M a whole number of 2 or more, and N no greater than the M beside it |
| N = on the Nelson rules 7 and 8 | a whole number of 1 or more |
| Sigma = on a Westgard rule | a number greater than 0 |
| LSL:, USL: and Target: on the Capability tab | a number, or blank, with LSL below both the others and USL above both. A box is compared only against a box that holds a value. |
| Sigma tolerance under the Custom capability standard | a number greater than 0 |
| Confidence level | greater than 0 and less than 1, and the message adds "It is a proportion, not a percent, so type 0.95 for 95 percent." |
| The histogram anchor value | a number, or blank |
| The histogram bin count | a whole number from 1 to 250 |
| The histogram bin width | a number of 0.001 or more, or blank |
| The split row box beside Add | a whole number of 1 or more, below the number of plotted points, and not already in the list |
This dialog has no Capability tab and no Transformation group, so the specification limit, target, sigma tolerance, confidence level, histogram and custom lambda rows above never arise on it. It shows only Mean: and Standard deviation (overall): under Historical Values, because it forms no within-subgroup or between-subgroup estimate.