![]() All the points need to be interpreted against the control limits but not specification limits. Once the R bar chart is in control, you can review the X bar chart and interpret the points against the control limits.Remove those subgroups from the calculations. Identify the special cause and address the issue. If the points are out of control in the R chart, then stop the process. If the values are out of control in the R chart that means that the X bar chart control limits are not accurate. The X bar chart control limits are derived from the R bar (average range) values. To correctly interpret the X Bar R chart, always examine the R chart first.For the first chart on a process, collect at least 20-25 sample groups to ensure that an adequate measure of process variation has been taken.The data should be collected in a random and consecutive manner.The sample size (sub-group) must be at least two and should not be more than 10-12.Some notes to keep in mind while creating an x-bar R chart are:.The centre line differs when subgroup sizes aren’t equal. The range statistic expected value is the centre line for each subgroup. Each point on a chart represents the subgroup range value. ![]() When measuring subgroups less than ten at regular intervals in a process, an R control chart can be used. The R-chart is used to monitor process variability like range.
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