Number Defective - Np Chart
This type of chart can be used when it is possible to establish a constant sample size n. It has the advantage that it is readily understood by users and easy to use. The numbers of defectives found in the samples are plotted directly. Again, the binomial distribution applies and the determination of control limits is similar in format to that for p-charts: n¯p is the expected number of defectives in a sample.
Warning Limits = n¯p + 1.96 √ n¯p (1- ¯p)
Action Limits = n¯p + 3.09 √ n¯p (1- ¯p)