To ensure that your results are valid, consider the following guidelines when you collect data, perform the analysis, and interpret your results.
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.
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.