NP chart plots the number of defectives and indicate the lack of control in the process. For a better understanding of this type of chart, let's work on the example for P chart as shown below:
Needed computations are: compute the proportion resulted by dividing the number of defectives to batch size (pi = xi/ni, where xi is the number of defectives, ni is the size of batch); compute the center line value, by multiplying each resulted proportion with batch size (ci = pi * ni); compute the lower and upper control limits as follow: LCL = ci – 3 * sqrt(ci * (1- pi)), UCL = ci + 3 * sqrt(ci * (1- pi)); if LCL is less than 0, set it to 0. If UCL is greater than ni, set it to ni; plot the data on the chart and observe if there are any points that fall outside the control limits. Resulted chart is shown above, near the table. Analyzing the chart is observed that no run fall outside the control limits, which is an indication of good control in the process. If we do the same alteration of data as in the example of P chart, by setting the defective number to 22 for run no. 4, the resulting chart shown above on the right. There is evidence that a lack of control does exist. Further investigation of causes of lack of control is needed.
Resulted chart is shown above, near the table. Analyzing the chart is observed that no run fall outside the control limits, which is an indication of good control in the process. If we do the same alteration of data as in the example of P chart, by setting the defective number to 22 for run no. 4, the resulting chart shown above on the right. There is evidence that a lack of control does exist. Further investigation of causes of lack of control is needed.
If we do the same alteration of data as in the example of