Click here to see how the timing of SPC ties into other quality tools. See the example in the graphic at the top of this web page. You want to make sure it is likely that there will be only a minimum of variation within each subgroup, so that variation between subgroups shows up on the chart. The subgroup average is plotted for on the top chart, and the range is plotted on the bottom chart. The R-bar value can also be used to calculate the Upper and Lower Control Limits for both charts.
Any plot points beyond the control limits suggest a special cause of variation.
In a process under statistical control, there should be about as many subgroup averages above the grand average line as there are below the line. There should not be too many points in a row above or below the grand average line, or too many in a row increasing or decreasing. The np chart requires a constant sample size, but it can vary with the p chart. This is because you will be using an estimate of the process standard deviation to calculate the process capability. It is not necessary, however, for the raw data to be normally distributed in order to use SPC charts.