np Chart A variation of the control chart for p is the np chart in which the actual numbers of defects are plotted instead of the proportions of defects. The np chart has a centerline value of np, and the control limits have values of np + 31np q and np - 31np q. The p chart and the np chart differ only in the scale of values used for the vertical axis. Construct the np chart for Example 1 in this section. Compare the np chart to the control chart for p given in this section.