To ensure that your results are valid, consider the following guidelines when you collect data, perform the analysis, and interpret your results.
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.
If you have fewer than 100 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 at least 100 data points.
If the process variation is not stable, the control limits may not adequately reflect the control status of the process.
If consecutive data points are correlated, the control limits will be too narrow and you may see a large number of false out-of-control signals.