It is actually a two plots to monitor the process mean and the process variation over the time and is an example of statistical process control. X bar R chart is used to monitor the process performance of a continuous data and the data to be collected in subgroups at a set time periods. Selection of appropriate control chart is very important in control charts mapping, otherwise ended up with inaccurate control limits for the data. In summary, use only control limits on control charts specification limits belong on histograms, box plots, and probability plots.X Bar R charts are the widely used control chart for variable data to examine the process stability in many industries (like Hospital patients’ blood pressure over time, customer call handle time, length of the part in production process etc.,). The items are all either too large or too small but on the average they're just right! But all of the measurements making up the subgroups (the "+"s) are outside the specifications. All of the subgroup averages (the "O"s) are within the specifications. The subgroup size is two, and the Target and Specifications (rather than Center Line and Control Limits) have been added to the chart. The only good use of specification limits on a control chart that I've seen is to make a point when the process is operating nowhere near the specifications.Ĭoncerning specifications appearing on an X-bar chart, the X-bar chart below illustrates the problem. The additional limits void the wonderful ability of control charts to give clear guidance. Additional limits risk confusing customer demands with process behavior. How about showing specifications on the control charts in addition to the control limits? The leading SPC solutions allow this, but it's generally not a good idea. The result is additional avoidable variation, lower quality, and higher costs. Signals of process change are ignored and opportunities for process improvement are missed. The control limits are set too loosely.Tampering adds to process variation, resulting in lower quality and higher costs. This leads to over-adjustment and tampering with the process. The control limits are set too tightly.Unless the specification and control limit values are identical, one of two errors occurs: Using specifications on an X-bar is the most egregious error: the specifications are in one unit (items) while the chart is in another (average of several items).Įven using specification limit values on an Individuals chart leads to problems.
The most common mistake is to use specification limit values instead of control limit values on an X-bar chart or an Individuals chart.
The table below contrasts control limits and specification limits: Control LimitsĬonfusing control limits with specification limits leads to mistakes. They generally apply to the individual items being measured and appear on histograms, box plots, or probability plots. Specification limits are chosen in numerous ways. An X-bar chart and an Individual measurements chart will have different limits. What is the relationship between control limits and specification limits? Usually there is no relationship whatsoever.Ĭontrol limits are calculated from process data for a particular control chart. But control limits and specification limits are completely different values and concepts. We often hear control limits and specification limits discussed as if they are interchangeable.
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