Home / Statistical Tools / Control Charts / u Chart / 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 the dialog behind all three U-family charts, because there is only one: the ribbon entry C, U, and Laney's U' opens a single window whose Variant: radio row selects c, u or Laney U'. Everything here applies to all three unless it says otherwise, and the two variant pages, c Chart Options and Laney U' Chart Options, list only their differences.
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¶
The dialog has three tabs: Data, Limits & Splits, and Rules & Display. Above them sits a Variant: row with C, U and Laney U'. U is selected when the dialog opens, even though the ribbon button names C first. The window title follows the selection, so the same dialog reads C Chart, U Chart or Laney U' Chart. The buttons along the bottom are Back, Help, Cancel and Finish.
There is no Capability tab, and no specification limit, target, capability index or histogram can reach an attribute chart's output. Also absent: the Transformation group and the Estimate Sigma group. A Poisson chart's sigma is fixed by its center line, so there is nothing to choose, and any historical sigma is ignored by the engine.
Data Tab¶
The calculation type sits at the top as Excel and GroupBy, with a Data Types button beside it. Below that:
| Control | Type | Default | What it does |
|---|---|---|---|
| Defects Column | slot list | empty | The count of defects per subgroup. Checking several columns produces several charts, one per column |
| Inspection Size | radio pair: Column or Constant | Constant | Where the area of opportunity comes from |
| n = | text box, live under Constant | 1 | The constant inspection size |
| X-Axis Labels (Optional) | slot list | empty | Labels drawn along the X axis |
| GroupBy | region with Available Items and GroupBy Order | empty | One chart per group |
Two things are specific to this dialog:
- The whole Inspection Size group is disabled when the variant is C. A c chart assumes every subgroup covered the same area of opportunity, so there is nothing to divide by; the group greys out rather than being ignored silently.
- There is no subgroup group. One row is one subgroup, and the inspection size says how large that subgroup's area of opportunity was. It does not have to be a whole number: 12.5 units, or 3.4 square metres, is fine.
A negative defect count or an inspection size of zero or less is refused, and the chart needs at least two usable subgroups.
Limits & Splits Tab¶
Three groups: Control Limits, Baseline Estimation and Split Control Limits.
Control Limits¶
| Control | Type | Default | What it does |
|---|---|---|---|
| Shewhart / Manual / None | radio group | Shewhart | Whether limits are computed, typed, or omitted |
| Number of Standard Deviations (k): | text box | 3 | The multiplier on the standard deviation of the plotted statistic |
| Historical Mean (optional): | text box | blank | Supplies the center line outright instead of estimating it |
| Manual UCL:, Manual Center:, Manual LCL: | text boxes | blank | Used only under Manual |
| Force Straight Limits | check box | off | Substitutes one assumed inspection size into the limits. Greyed out on the C variant, and on U and Laney U' while Inspection Size is Constant |
| Assumed n: | text box | blank | The size used when Force Straight Limits is on. Greyed out under the same condition |
Force Straight Limits behaves differently here than on the np chart, and the difference follows from what varies on each chart:
| Variant | What an assumed n changes |
|---|---|
| C | nothing; the Inspection Size group is disabled on C, and the limits are already straight |
| U | the limits only. The center line is \(\bar{u}\), which involves no single subgroup's size, so it was already flat and does not move |
| Laney U' | the limits only, the same as U |
So on a u chart this control turns a stepped band into a straight one without touching the center line. On an np chart the equivalent control also flattens the center line, because an np chart's center line steps and a u chart's does not.
Manual limits replace the lines with the numbers you type, and all three boxes are optional. Enter all three, in increasing order, and that band and center line are drawn on every point. Enter Manual UCL: and Manual LCL: only and both limits are drawn with no center line, which silences the tests that count runs relative to the center. Errors are reported in red under the boxes and Finish stays disabled until they are fixed. None draws the points and the center line with no limits, which leaves the out-of-control tests nothing to compare against.
Baseline Estimation¶
| Control | Type | Default | What it does |
|---|---|---|---|
| All Subgroups | radio button | selected | Estimates \(\bar{u}\) from every subgroup |
| First N Subgroups | radio button | not selected | Estimates from the first N only |
The count is a count of subgroups. Every subgroup is still plotted; only the estimate is restricted, and the resulting limits extend across the whole chart.
The count must be a whole number of 4 or more. Type less and the box refuses it with "The first N subgroups must be a whole number of 4 or more." and returns to its previous value.
Four is one minimum for all four of these dialogs, and it is deliberately stricter than the engines behind them, which stop in different places: the G and T calculations need three included values because each estimates a distribution parameter, while the p, np, u and Laney calculations need two because they only pool counts over sample sizes. On top of that, a G or T chart reading a column of dates and times works on the gaps between the dates, so four dates give three gaps. One minimum of 4 satisfies every one of those, and it means the box no longer accepts a number that Finish would then reject.
Split Control Limits¶
| Control | Type | Default | What it does |
|---|---|---|---|
| None | radio button | selected | One estimate for the whole chart |
| Column | radio button | not selected | A new phase begins wherever the chosen column's value changes |
| Manual Breaks | radio button | not selected | A new phase begins at row numbers you list |
| Split Column: | drop-down | empty | The column read under Column mode |
| Manual Break Rows (1-based, start of split): | text box + Add / Remove Selected | empty | The list of phase starting rows |
| Split Labels (shown beside each divider): | grid | empty | One editable label per phase |
Each phase gets its own \(\bar{u}\), its own limits and its own \(\sigma_Z\), and the summary table gains a Per-Split Breakdown block.
Rules & Display Tab¶
Out-of-Control Rules¶
Only Rule 1 is on when the dialog opens.
| Control | Default | Threshold box |
|---|---|---|
| Rule 1 - Beyond K sigma from center | on | none |
| Rule 2 - N consecutive points on same side of center | off | N = 9 |
| Rule 3 - N consecutive increasing or decreasing | off | N = 6 |
| Rule 4 - N consecutive alternating up/down | off | N = 14 |
Rules 5 through 8 are not on this dialog and are disabled in the engine, so the zone-reading tests cannot run on an attribute chart. Rule 9, the zero-count Benneyan test, belongs to the G Chart, which is worth knowing because a u chart can legitimately plot a rate of zero and no rule here will treat a run of zeros as a signal.
Each threshold box takes a whole number of 2 or more.
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¶
| Control | Type | Default | What it does |
|---|---|---|---|
| Reference Lines: | grid with Value, Label, Color, Thickness, plus Add Reference Line and Apply | empty | Draws a horizontal line at a value you choose |
There is no zone line option on this dialog, and this chart draws no zone lines. The control limits are not symmetric about the center line, so a zone measured from the upper half of the band does not describe the lower half. The engine still computes the four values, and the construction is on the Math Details page, but nothing is plotted, and the four tests that read zones (rules 5 through 8) are not offered on an attribute chart in any case.
A reference line on a u chart is drawn in defects per unit.
Diagnostics¶
This group exists only on the attribute dialogs.
| Control | Type | Default | What it does |
|---|---|---|---|
| Compute and report sigma_Z statistic | check box | on | Computes the overdispersion factor and writes it to the summary table |
| Include secondary Z-score plot | check box | off | Adds a plot of the standardized subgroup values |
\(\sigma_Z\) is reported on all three variants and applied by Laney U' alone. On a c or u chart it is advice; on a Laney U' chart it is the arithmetic. See Math Details.
Task Pane¶
The Control Chart pane that opens beside a finished chart has its own page: Task Pane.
Changing a column's Data Type can take it out of a slot, and the dialog says so. The column lists on this tab hold only the columns their slot accepts. Change a column's type in the Data Types list to something the slot no longer accepts and the column leaves that slot, even if it was ticked, and a message appears naming the column, its new type, the slot it left, and what that slot does accept:
The column 'Rejects' is set to Continuous, so it has been removed from Defectives, which accepts Count.
The slots and what each one accepts: Defectives and Sample Size on a p chart take Count; Defects on a u chart takes Count, and Inspection Size takes Continuous, Integer or Count; Data on a G chart takes Count or DateTime.
If the column leaves more than one slot, a second sentence follows: "It has also been removed from slot, which accepts types." The chart is then built without that column in that role, so read the message rather than pressing Finish and wondering where the series went.
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.
These boxes on this dialog refuse with a message:
| Box | What it must be |
|---|---|
| Number of Standard Deviations (k): | a number greater than 0 |
| Historical Mean (optional): | a number greater than 0 |
| First N Subgroups count | a whole number of 4 or more |
| N = on rules 2, 3 and 4 | a whole number of 2 or more. The message names the rule: "The rule 2 number of points must be a whole number of 2 or more." |
| Value in the Reference Lines grid | a number of 0 or more. A u chart plots a rate, so there is nothing below zero to draw a line at. |
n = under Constant inspection size |
a number greater than 0. Fractions are allowed, because an area of opportunity may be half a square metre. Earlier versions replaced an unusable size with 1 without saying so. |
| Assumed n: under Force Straight Limits | a number greater than 0 |
| Manual UCL, Center and LCL | 0 or more, and in increasing order. Each box is also compared with the others in its row, so "The manual UCL must be greater than the manual center." names both. A box is compared only against a box that holds a value, so a row can be filled in any order. |