Remove those subgroups from the calculations. If the points are out of control in the R chart, then stop the process, identify the special cause, and address the issue.If the values are out of control in the R chart, then the X-bar chart control limits may not be accurate. The X-bar chart control limits are derived from the R bar (average range) values.To correctly interpret X-bar/R charts, always examine the R chart first.Here are the guidelines for interpreting them: Once you have constructed your X-bar/R charts, you’ll need to interpret what they are telling you about your process. Do not artificially subgroup data to dampen or reduce apparent variation. There must be some logic and rationale for the subgroups. The purpose of subgrouping is to minimize the within -sample variation so as to maximize the ability to see between -sample variation.ĭo not create subgroups for convenience. Rational subgrouping is the process of selecting a subgroup based upon logical grouping criteria or statistical considerations.
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