X bar is the average reading in a sample.Identify the appropriate Measurement System Evaluation (MSE). Otherwise, if all the values are within the control limits, it may be because of the slop in the measurement system, and the team won’t focus on it. Sometimes, having a few points out of control on the x-bar portion would also be good in the initial phase.Also, you should remove the subgroup before the calculation. The process is to be in control in the early phase of production. You must identify any special causes if any of the points are out of control during the initial phase.These values are to be considered for creating control limits for both ranges and the mean of each subgroup. The first set of subgroups determines the process mean and standard deviation.In Analyze phase, they collected 20 sets of plate thickness samples with a subgroup size of 4. During the Measure phase, the project team performed the process capability study and identified that the process was not capable (less than 2 sigmas). Collect a minimum of 20 to 25 sets of samples in the time sequenceĮxample: In the manufacturing industry, plate thickness is one of the important CTQ factors.Choose the appropriate subgroup size and the sampling frequency.Determine the objective of the chart and choose the important variables.Steps to follow for X bar R chart The objective of the chart and subgroup size There is no need to perform process capability studies for an unstable process. Process capability studies can only be performed after the X bar and R chart values are within the control limits.As the customer or management provides specification limits, whereas control limits are derived from the average and range values of the subgroups. All the points need to be interpreted against the control limits but not specification limits.Once the R bar chart is in control, then 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 R chart’s values are out of control, the X bar chart control limits are inaccurate. The X bar chart controls limits that are derived from the R bar (average range) values.To correctly interpret an X bar R chart, always examine the R chart first.How to Interpret the X Bar R Control Charts When the collected data is continuous (i.e., Length, Weight), etc., and captures data in time order.The X Bar S Control chart is to be considered when the subgroup size is more than 10.An X bar R chart is for a subgroup size of more than one (for an I-MR chart the subgroup size is one only), and generally, it is used when rationally collecting measurements in a subgroup size between two and ten observations.When the data is assumed to be normally distributed.An X bar R chart will help to identify the process variation over time. Even a very stable process may have some minor variations, which will cause the process instability.This monitors the spread of the process over time. R chart: The range of the process over time from subgroups values. The control limits on the X bar consider the sample’s mean and center. X bar chart: The mean or average change in a process over time from subgroup values. They consider the previous value means at each point. The cumulative sum ( CUSUM) and the exponentially weighted moving average ( EWMA) charts also monitor the mean of the process, but the basic difference is unlike the X bar chart. These combination charts help to understand the stability of processes and detect special cause variation. It is actually two plots to monitor the process mean and the process variation over time and is an example of statistical process control. You can also use them to collect data from subgroups at set time periods. X bar R chart is used to monitor the process performance of continuous data. Selection of the appropriate control chart is very important in control chart mapping otherwise, you will end up with inaccurate control limits for the data. X Bar R charts are the widely used control charts for variable data to examine the process stability in many industries (like hospital patients’ blood pressure over time, customer call handle times, length of a part in a production process, etc).
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