Home / Statistical Tools / Control Charts / np Chart / Options
Options¶
The np chart shares its dialog with the p chart and the Laney P' chart. One ribbon entry, P, NP, and Laney's P', opens a single window, and the Variant: radio row at the top selects which chart you get. Every control is documented once, on p Chart Options. This page lists only what is different when the variant is NP.
What changes when you select NP¶
| Window title | NP Chart |
| Worksheet name | NP Chart, or NP Chart - {GroupName} under GroupBy |
| Y axis | Count Defective, where the p chart says Proportion Defective |
| Plotted statistic | the raw count of defectives, not the fraction |
Nothing is added to the dialog and nothing is removed from it. The slots, the limit types, the baseline and split groups, the four rules, the display options and the diagnostics are all the ones described on p Chart Options.
The one control that behaves differently¶
Force Straight Limits is live on this variant and inert on the other two.
| Variant | What an assumed n changes |
|---|---|
| P | nothing |
| NP | the center line and the limits |
| Laney P' | nothing |
The reason is in what the two charts draw. A p chart's center line is \(\bar{p}\), which does not involve a sample size at all, so there is nothing for an assumed size to replace. An np chart's center line is \(n_i\bar{p}\), so with varying sample sizes it steps, and substituting a single assumed \(n\) flattens the center line and the limits together.
That makes this control more than cosmetic here: without it, an np chart of unequal subgroups has a staircase center line, and a reader comparing a point to the line has to know which step it belongs to.
Reference values it is worth knowing¶
- The upper limit is capped at the sample size on this variant, where the p chart caps at 1. Both are the same constraint expressed in the chart's own units.
- The lower limit is floored at 0 on both.
- \(\sigma_Z\) is computed and reported on this variant exactly as on the other two, and applied on none of them but Laney P'.