Data considerations for Xbar Chart

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

The data should be continuous

If your data are counts of defectives or defects, use an attribute control chart, such as P Chart or U Chart.

The data should be in time order

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.

The data should be collected at appropriate time intervals

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.

The data should be in rational subgroups

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 do not have subgroups, use Individuals Chart.

Collect an appropriate amount of data

The appropriate amount of data depends on the subgroup size.

• If the subgroup size is less than or equal to 2, collect at least 100 observations.
• If the subgroup size is 3, collect at least 80 observations.
• If the subgroup size is 4 or 5, collect at least 70 observations.
• If the subgroup size is 6 or greater, collect at least 60 observations.

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.

The process variation should be stable

If the process variation is not stable, the control limits may not adequately reflect the control status of the process.

The data do not need to be normally distributed

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.

The observations within each subgroup should not be correlated with each other

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.