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np Chart

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QXL Stat Tools Tab > Control Charts > np Chart

An np chart monitors the number of defective items in each subgroup. Like the p chart it is an attribute chart for pass/fail data, but it plots the raw count of defectives rather than the proportion. It is most natural when the subgroup size is the same for every sample.

When to use it

  • You want to see the count of defective items per subgroup rather than the proportion.
  • Subgroup sizes are constant (or nearly so). When sizes vary, the p Chart is usually easier to read because its center line stays flat.
  • If the points are overdispersed relative to the binomial model, prefer the Laney P' Chart; the np chart reports the σ_Z diagnostic to help you decide.

How it works

The center line is the expected number of defectives, the subgroup size times the pooled proportion defective. The plotted point is the raw count of defectives in the subgroup. The control limits sit three standard deviations from the center line by default, with the lower limit held at zero and the upper limit capped at the subgroup size. When subgroup sizes vary, both the center line and the limits move from point to point; the Force Straight Limits option can substitute a single assumed size to keep them level. The exact formulas are on the Math Details page.

Options

Data

  • Data source: an Excel range, or GroupBy (each value of the grouping column produces a separate chart).
  • Defectives column: the count of defective items; selecting several columns produces several charts.
  • Sample size: taken from a column or entered as a constant.
  • X-axis labels (optional): a column of dates, text, or numbers to label the points.

Control limits

  • Limit type: Shewhart (calculated), Manual (you enter the upper limit, center, and lower limit), or None (center line only).
  • Number of standard deviations: the sigma multiplier for the limits (default 3).
  • Historical proportion: supply a known proportion defective instead of estimating it from the data.
  • Force straight limits: substitute a single assumed subgroup size so the center line and limits are level (available when the sample size comes from a column).
  • Baseline subgroups: estimate the center from all subgroups or from the first N subgroups.
  • Split control limits: break the chart into phases with separate limits, split at the changes in a column's value or at manual break rows, with an optional label per phase.

Tests and display

  • Out-of-control tests: the point-pattern rules that flag signals; attribute charts use rules 1 through 4, and the run length for rules 2, 3, and 4 is adjustable. See Out-of-Control tests.
  • Zones: show the one- and two-sigma zone lines.
  • Reference lines: add horizontal lines at chosen values, each with a label, color, and thickness.
  • Overdispersion diagnostic: compute the σ_Z statistic, and optionally a secondary Z-score probability plot, to judge whether the Laney P' chart is warranted.

After creation

The Control Chart task pane lets you scroll and zoom the visible window, toggle the zone, out-of-control, and outlier markers, and edit the phase splits.

For the shared steps of building and updating a control chart, see Create Control Charts and Data Sources.

Output

Quantum XL draws the np chart and a summary listing the center line, the control limits, the number of subgroups, the count of out-of-control points, and the σ_Z overdispersion diagnostic.

See Also