Home / Statistical Tools / Control Charts / p 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 P-family charts, because there is only one: the ribbon entry P, NP, and Laney's P' opens a single window whose Variant: radio row selects p, np or Laney P'. Everything here applies to all three unless it says otherwise, and the two variant pages, np Chart Options and Laney P' 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 P, NP and Laney P'; P is selected when the dialog opens. The window title follows the selection, so the same dialog reads P Chart, NP Chart or Laney P' 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. An attribute chart's sigma is not estimated separately from its center line: the binomial model ties them together, so there is nothing to choose. Any historical sigma is ignored by the engine even if some other part of the product offers one.
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 |
|---|---|---|---|
| Defectives Column | slot list | empty | The count of defective items per subgroup. Checking several columns produces several charts, one per column |
| Sample Size | radio pair: Column or Constant | Constant | Where the number inspected comes from |
| n = | text box, live under Constant | 1000, or the largest value in the Defectives column when that exceeds 1000 | The constant sample 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 |
Three things are worth knowing before you fill this in:
- The sample size default is a constant, normally 1000, not a column. If your sample sizes live in a column, switch to Column first; leaving it on Constant silently charts every subgroup as though 1000 items had been inspected.
- There is no subgroup group on this dialog. One row is one subgroup, and the sample size column says how many items that subgroup contained.
- Defectives cannot exceed the sample size. One offending row stops the analysis with a message naming the subgroup number. A negative count or a sample size of zero or less is refused the same way, 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 p (optional): | text box | blank | Supplies \(\bar{p}\) 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 sample size for the real ones. Greyed out unless the variant is NP and Sample Size is set to Column |
| Assumed n: | text box | blank | The size used when Force Straight Limits is on. Greyed out under the same condition |
Historical p must be a proportion greater than 0 and less than 1, on every variant including np. Supplied, it replaces the pooled estimate and the data no longer influences where the center line sits.
Force Straight Limits does nothing on a p chart. This is the one control on the tab that differs by variant, and the difference is not cosmetic:
| Variant | What an assumed n changes |
|---|---|
| P | Nothing. The engine never substitutes an assumed size on this variant, so the limits keep stepping with the real sample sizes |
| NP | The center line and the limits, both of which otherwise vary with the sample size |
| Laney P' | Nothing, for the same reason as P |
So on a p chart the check box and its Assumed n: box are visible but greyed out, and cannot be set at all. They come alive only on the NP variant, and only when Sample Size is read from a Column rather than entered as a Constant.
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, and with no limits there is nothing for the out-of-control tests to compare against.
Baseline Estimation¶
| Control | Type | Default | What it does |
|---|---|---|---|
| All Subgroups | radio button | selected | Estimates \(\bar{p}\) 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 \(\bar{p}\), 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. The variable charts run all eight tests by default; these run one.
| 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 be run on an attribute chart at all. Rule 9, the zero-count Benneyan test, belongs to the G Chart and is not here either.
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 p chart is drawn in proportions, so a target of 2% is entered as 0.02.
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 P' alone. On a p chart it is advice; on a Laney P' 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 p (optional): | greater than 0 and less than 1, and the message adds "It is a proportion, not a percent, so type 0.05 for 5 percent." |
| 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 p chart plots a proportion and an np chart a count, so there is nothing below zero to draw a line at. |
n = under Constant sample size |
a whole number of 1 or more |
| Assumed n: under Force Straight Limits | a whole number of 1 or more. With the checkbox ticked, Finish stays off until this box holds a size. |
| Manual UCL, Center and LCL | on the p and Laney P' variants, between 0 and 1, because those charts plot a proportion; on the np variant, 0 or more, because it plots a count. 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. |
The historical box is called Historical p on every variant¶
It used to read Historical pBar (optional):. It is now Historical p (optional): on all three variants, including np. The np chart plots a count, but this box still takes the proportion: the chart multiplies it by the sample size itself, so typing the count you expect would place the center line far too high.
A manual limit typed under np, then carried to p¶
Manual limits survive a variant switch, and a limit that is legal under np is not legal under p or Laney P'. Type a manual UCL above 1 while the variant is np, then switch to p or Laney P', and the chart stops redrawing, Finish switches off, and a message box opens:
The manual UCL must be between 0 and 1 on a P chart; it was typed under NP.
Nothing you typed is removed and the variant switch is not undone. Change the limit, or go back to np, and the chart redraws. Before this, the switch ran the analysis with the out-of-range limit still in place.