Control limits for the p chart are typically placed so that they are 3 standard errors above and below the centerline.

Question

Quizplus · Accepted Answer

This is true for the Shewhart p chart, which is the most commonly used type of p chart. The control limits are based on the assumption of a binomial distribution for the data and are calculated using the formula:

Upper Control Limit = p̂ + 3 √(p̂(1-p̂)/n)
Lower Control Limit = p̂ - 3 √(p̂(1-p̂)/n)

where p̂ is the overall proportion of nonconforming items, n is the sample size, and √ is the square root function. The control limits are usually set at ±3 standard errors from the centerline, which is the sample proportion p̂. This provides a 99.73% probability of detecting a shift in the process mean.

Control Limits for the P Chart Are Typically Placed So