P Chart

In this chart, we plot the percent of defectives (per batch, per day, per machine, etc.) as in the U chart. However, the control limits in this chart are not based on the distribution of rare events but rather on the binomial distribution (of proportions). Therefore, this chart is most applicable to situations where the occurrence of defectives is not rare (e.g., we expect the percent of defectives to be more than 5% of the total number of units produced).

The P chart is described in most standard quality control textbooks; see for example, Hart and Hart (1989), Juran (1962), Juran and Gryna (1970), Montgomery (1985, 1991), Pyzdek (1989), or Vaughn (1974).