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.
If you do not want to detect small shifts in a process, use a variables chart for subgroups, such as Xbar-R Chart, or an variables chart for individuals, such as I-MR Chart.
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.
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.