To ensure that your results are valid, consider the following guidelines when you collect data, perform the analysis, and interpret your results.
If your data are counts of defectives or defects, use an attribute control chart, such as P Chart or U Chart.
Because control charts detect changes over time, the order of the data is important. You should enter the data in the order it was collected, with the oldest data at the top of the worksheet.
Collect data at equally spaced time intervals, such as every hour, every shift, or every day. Select a time interval that is short enough that you can identify changes to the process soon after the changes occur.
A rational subgroup is a small sample of items that are similar, that are produced in a short period of time under the same conditions (such as operator, equipment, or supplier), and that are representative of the output from a process. If the subgroups are not rational subgroups, the estimated control limits might be too wide.
If you have fewer than the recommended number of observations, then you can still use the control chart, but the results are preliminary because the control limits may not be precise. If you use the chart regularly, re-estimate the standard deviation and the control limits after you collect the recommended number of observations.
Although most control charts for variables data are formally based on the assumption of normality, you can still obtain good results with nonnormal data if you collect data in subgroups. The required subgroup size depends on how nonnormal the data are.
If consecutive data points within each subgroup are correlated, the control limits will be too narrow, and the control chart might incorrectly show some in-control points as out of control.