Home / Statistical Tools / Control Charts / T 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 T Chart dialog. It is not built from the same options kit as the variable charts, so several groups you may expect are absent rather than disabled, and each absence is called out below at the point where you would look for it. 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 is titled T Chart and has three tabs: Data, Limits & Splits, and Rules & Display. There is no Chart: variant row above them: one panel, no variants. The buttons along the bottom are Back, Help, Cancel and Finish.
There is no Capability tab, and that is not a hidden tab: no specification limit, no target, no capability index and no histogram can reach a T chart's output, in the engine as well as in the dialog.
Also absent: the Transformation group, the Control Limits limit-type radios and manual limits grid, the usual five-box Historical Values group, and the Estimate Sigma group. What replaces the last two is the distribution fitting described below.
Data Tab¶
The calculation type sits at the top as Excel and GroupBy, with a Data Types button beside it. Below that are two slot lists, Data Columns (required) and X-Axis Labels (Optional), and under GroupBy a region with Available Items and GroupBy Order.
Four things are specific to this chart:
- Four data types are accepted: Continuous, Integer, Count and DateTime. That is wider than the G chart, which takes only Count and DateTime, and it is the point of the T chart: the interval can be a measurement with decimals.
- Checking more than one data column produces more than one chart, one per column. There is no subgroup concept: one row is one event.
- The minimum is 3 intervals, or 4 date-times. Four stamps produce three intervals, because the first only establishes the starting point.
- A DateTime column gets a Time Unit drop-down of its own, beside the column in the Data Columns list. It appears only for DateTime columns.
Intervals or date-times¶
An interval column holds the elapsed times directly, in whatever unit you measured.
A DateTime column holds the moment each event occurred, and Quantum XL does the subtraction. Three details matter:
- The Time Unit drop-down converts the difference: Days, Hours, Minutes or Seconds, with Days the default. The factors are exactly what they look like: one day, a twenty-fourth of a day, a 1440th, an 86400th.
- The result keeps its fraction. Nothing is truncated, so two events 3.9 days apart contribute 3.9 days, or 93.6 hours. This is the difference that separates this chart from the G Chart, which floors the same difference to 3.
- The stamps must be in chronological order. One out-of-order row stops the analysis with a message naming both row numbers and both dates. And the first stamp produces no point, so N stamps give N minus 1 intervals.
The Y-axis title follows the unit you chose: Days Between Events, Hours Between Events and so on, or plain Time Between Events when the column was numeric.
Limits & Splits Tab¶
Three groups: Control Limits, Baseline Estimation and Split Control Limits.
Control Limits¶
| Control | Type | Default | What it does |
|---|---|---|---|
| Number of Standard Deviations (k): | text box | 3 | Selects which quantiles of the fitted distribution the lines are drawn at |
| Fit exponential distribution only | check box | off | Fixes the shape at 1 and makes the scale the sample mean |
| Historical Weibull α (scale): | text box | blank | Supplies the scale outright |
| Historical Weibull β (shape): | text box | blank | Supplies the shape outright |
The sigma multiplier does not become a distance. There is no standard deviation being multiplied. The number is converted into tail probabilities through the normal distribution and the lines are the Weibull quantiles at those probabilities: at the default 3, the 0.135th and 99.865th percentiles. It must be greater than zero.
Fit exponential distribution only is the setting to understand first, because it changes what the chart is capable of telling you and it also changes how reproducible the numbers are:
- Cleared, the default, both Weibull parameters are estimated from your intervals by an iterative fit. A shape above 1 means the longer you wait the more likely the next event becomes; below 1, the opposite. That is real information a rare-event record can carry.
- Ticked, the shape is 1 and the scale is the plain average of your intervals. Every line on the chart is then a multiple of that average, and you can check the whole chart with a calculator.
Ticking the box clears and disables the Historical Weibull β box, since the shape is being forced to 1. The α box stays available, so supplying a scale while the shape is forced to 1 is a valid combination: the chart is then fully determined by one number you typed.
In general-Weibull mode the two historical boxes are all or nothing: supply both and neither is estimated, supply neither and both are estimated, supply exactly one and the analysis stops with an invalid-override error. Either value must be greater than zero.
Baseline Estimation¶
| Control | Type | Default | What it does |
|---|---|---|---|
| All Subgroups | radio button | selected | Fits from every event |
| First N Subgroups | radio button | not selected | Fits from the first N events only |
The count is a count of events, not worksheet rows, and only the fit uses it: every interval is still plotted. The lines from those first N events extend across the whole chart.
Supplying both historical parameters makes this group irrelevant, because nothing is being fitted.
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 set of limits 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 is fitted separately, so every phase gets its own shape, scale, median and limits. A divider is drawn at each boundary with the label you typed, and the summary table gains a Per-Phase Breakdown block. A phase needs enough events to fit on its own, which is the practical limit on how finely a rare-event record can be split.
Rules & Display Tab¶
Out-of-Control Rules¶
Only Rule 1 is on when the dialog opens, the opposite of the variable charts.
| 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 |
Rule 1 here means "outside the drawn limits", and because the lower limit sits a hair above zero while intervals are almost always larger than that, in practice it fires only upward: a wait longer than the model expects.
There is no Rule 9 on this dialog. The zero-count Benneyan test belongs to the G Chart, where a count of zero is a meaningful value. Rules 5 through 8 are not offered either and are disabled in the engine for this chart.
Every threshold box accepts a whole number of 2 or more. A smaller value is refused with a message box naming the rule, for example "The rule 2 number of points must be a whole number of 2 or more.", and the box goes back to the value it held before. The G chart now does the same.
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 distribution behind the limits is skewed, so a line a third of the way to the limit is not a standard deviation and the two sides are not the same distance from the center; four lines labelled 1 sigma and 2 sigma would say something untrue. The four values are still computed, and the construction is on the Math Details page, but nothing is plotted and no out-of-control test on this chart reads them.
A reference line is drawn in the chart's own units, which are the units of your intervals, or the Time Unit you selected for a DateTime column.
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.
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 Weibull α (scale): | a number greater than 0 |
| Historical Weibull β (shape): | 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 3 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 T chart plots an elapsed interval, so there is nothing below zero to draw a line at. |