Home / Statistical Tools / Control Charts / Laney U' Chart / Options
Options¶
The Laney U' chart shares its dialog with the c chart and the u chart. One ribbon entry, C, U, and Laney's U', opens a single window, and the Variant: radio row at the top selects which chart you get. Every control is documented once, on u Chart Options. This page lists only what is different when the variant is Laney U'.
What changes when you select Laney U'¶
| Window title | Laney U' Chart |
| Worksheet name | Laney U Chart, without the apostrophe, or Laney U Chart - {GroupName} under GroupBy |
| Y axis | Defect Rate, the same as the u chart |
| Plotted statistic | defects over inspection size, the same as the u chart |
| Limits | every subgroup's standard deviation multiplied by \(\sigma_Z\) before the limits are placed |
Nothing is added to the dialog and nothing is removed from it. The variant changes what the engine does with the numbers, not what the window offers.
The controls whose meaning shifts on this variant¶
Diagnostics: sigma_Z stops being advice¶
On a c or u chart, Compute and report sigma_Z statistic produces a number for the summary table and nothing else. On this variant the same number is the limit calculation. It is on by default.
With the box on, this chart cannot be built without \(\sigma_Z\), because \(\sigma_Z\) is not optional to it. If the estimate cannot be formed at all, from too few usable subgroups, the engine falls back to a factor of 1, and what you get is a u chart wearing a Laney U' title. Read the reported figure rather than assuming a correction was applied.
Inspection Size: still needed, and a constant is allowed¶
The Inspection Size group is live on this variant, as it is on u and unlike on c. It can be a column or a Constant. A constant does not disable the correction: it makes every subgroup's standard deviation the same, so the band is straight, and \(\sigma_Z\) still multiplies it.
Force Straight Limits: live here, unlike on Laney P'¶
| Variant | What an assumed n changes |
|---|---|
| C | nothing; the limits are already straight |
| U | the limits only |
| Laney U' | the limits only |
This is a real difference between the two Laney charts. On Laney P' an assumed n does nothing at all, because the engine never substitutes one on that variant. Here it does, so a Laney U' band can be both widened by \(\sigma_Z\) and flattened by an assumed size.
Manual and None limit types: they bypass the correction¶
Under Manual, all three lines are the numbers you typed, so \(\sigma_Z\) plays no part. Under None, no limits are drawn and \(\sigma_Z\) is not computed. Both are available on the plain charts too, but here the loss is larger, because the correction is the reason to be on this variant.
Split Control Limits: a sigma_Z per phase¶
Splitting re-estimates the correction inside each phase, so each phase carries its own \(\sigma_Z\) and its own band width, and the Per-Split Breakdown block reports them separately. If the overdispersion came from a step change partway through the record, a split can return phases whose \(\sigma_Z\) values are near 1.