Data considerations for I-MR-R/S 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 are in subgroups that have a consistent source of internal variation

It is important to make sure that measurements are taken in the same set of locations for each subgroup so that the internal variation is consistent.

The data should include at least 100 total observations across all subgroups

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 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.

By using this site you agree to the use of cookies for analytics and personalized content.  Read our policy