The chart is used in the following way. Periodically, a random sample of size n is taken, and the number of defective services or products is counted. The number of defectives is divided by the sample size to get a sample proportion defective, p, which is plotted on the chart. When a sample proportion defective falls outside the control limits, the analyst assumes that the proportion defective generated by the process has changed and searches for the assignable cause. Observations falling below the LCLp indicate that the process may actually have improved. The analyst may find no assignable cause because it is always possible that an out-of-control proportion occurred randomly.However, if the analyst discovers assignable causes, those sample data should not be used to calculate the control limits for the chart.