Each plotted point represents the proportion of defectives for one subgroup, pi:
Data points on a P chart follow the binomial distribution.
number of defectives for subgroup i
size of subgroup i
The center line represents the proportion of defectives for your process, . If you do not specify a historical value, then Minitab uses the average proportion of defectives from your data, , to estimate .
number of defectives in subgroup
size of subgroup
Lower control limit (LCL)
The LCL for each subgroup is equal to the greater of the following:
Upper control limit (UCL)
The UCL for each subgroup is equal to the lesser of the following: