P-chart

In industrial statistics, the p-chart is a type of control chart that is very similar to the X-bar chart except that the statistic being plotted is the sample proportion rather than the sample mean. Since the proportion deals with the percentage of successes, clearly the appropriate data for p-charts needs to be attribute data where the outcomes for each trial can be classified as either a success or a failure (conform or non-conform, yes or no, etc.). The subgroup size should ideally be equal, although unequal sample sizes can be accommodated.

p-chart


 * 1) The "p" comes from use of the proportion of nonconforming items
 * 2) Need a good definition of nonconforming items – usually a categorical definition
 * 3) Can be of equal or unequal subgroups
 * 4) Normally need large subgroups – can even be up to total for the period

Control limits for the p-chart are calculated on the basis of the binomial distribution.

The control limits for this chart type can be determined by the formula: $$\bar p \pm 3\sqrt{\frac{\bar p(1-\bar p)}{n}}$$