P-chart
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The P chart 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 must be equal so the proportion can be determined by dividing the outcome by the subgroup size.
P-chart
- The “p” comes from use of the proportion of nonconforming items
- Need a good definition of nonconforming items – usually a categorical definition
- Can of equal or unequal subgroups
- Normally need large subgroups – can even be to total for the period
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the subgroup number are not necessarily equal.
