Home / Statistical Tools / Control Charts / c Chart / Options
Options¶
The c chart shares its dialog with the u chart and the Laney 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 C.
What changes when you select C¶
| Window title | C Chart |
| Worksheet name | C Chart, or C Chart - {GroupName} under GroupBy |
| Y axis | Count of Defects, where the u chart says Defect Rate |
| Plotted statistic | the raw defect count, not the rate |
| Limits | straight lines, the only attribute chart where they do not step |
You have to select this variant deliberately. The dialog opens on U, even though the ribbon button names C first.
The one group that is switched off¶
The whole Inspection Size group is disabled on this variant. Both the Column and Constant radios and the n = box grey out together. A c chart assumes every subgroup covered the same area of opportunity, so there is nothing to divide by and nothing to supply.
That has two consequences worth stating:
- You cannot accidentally build a c chart from unequal subgroups by leaving the size behind, because the control is not available to leave behind. What you can do is select the C variant when your subgroups actually differ in size, and the chart will then compare counts that are not comparable. Nothing in the dialog can detect that; the data column looks the same either way.
- Force Straight Limits does nothing here. The limits are already straight, because the single standard deviation \(\sqrt{\bar{c}}\) does not depend on a subgroup size. The box can be ticked and an Assumed n: typed, and the chart will not change.
Everything else, in one table¶
| Control | Behaviour on the C variant |
|---|---|
| Number of StDevs: | as documented on u Chart Options |
| Historical uBar (optional): | supplies \(\bar{c}\) directly; the box is shared across the family, so the label says uBar |
| Shewhart / Manual / None | as documented; Manual replaces all three straight lines |
| Baseline Estimation | restricts which subgroups \(\bar{c}\) is averaged over |
| Split Control Limits | each phase gets its own \(\bar{c}\), so a split chart has straight limits per phase at different heights |
| Rules 1 to 4 | Rule 1 on, the rest off. Rules 5 to 8 are not offered |
| Zone lines | one full standard deviation apart, and on this chart that means four more straight lines |
| Diagnostics | \(\sigma_Z\) computed and reported, never applied. Applying it is the Laney U' Chart |