Minitab displays the confidence interval of the % of defective items, which indicates the precision of the estimate. It is generally recommended that you collect at least 25 subgroups over a long enough period of time to capture the different sources of process variation. The number of subgroups required depends on the average number of defective items and on the subgroup size. To obtain accurate estimates, you must collect enough subgroups. However, if the call center tracks all of the incoming calls during a randomly selected hour of the day, the number of calls is likely to vary and result in unequal subgroup sizes. For example, if a call center tracks 100 incoming calls each hour and counts the number of unsatisfactory wait times, all of the subgroup sizes are 100. Minitab checks that the subgroup size is large enough based on your data and reports the subgroup size that is needed to produce a reliable control chart. If some subgroup sizes are too small, you cannot adequately assess process stability. The items in each subgroup should be collected under the same inputs and conditions, such as personnel, equipment, suppliers, or environment. Investigate out-of-control points and eliminate any special cause variation in your process before continuing with the capability analysis.Ĭollect data in subgroups (samples, lots).Ī subgroup is a collection of similar items that are representative of the output from the process you want to evaluate. In the Diagnostic Report, Minitab displays a P chart that you can use to determine whether your process is stable. If it is not, you cannot accurately predict future capability. When you use data from the current process to predict future performance, the current process must be stable and in control. Process capability determines the capability of the current process and can also be used to predict the future, ongoing capability of the process. Make sure to read to guidelines, they are very informative.īinomial capability analysis determines whether the % defective meets customer requirements. If you are using Minitab 16, there is a very nice step by step guide on how to do capability analysis for binomial data.Īssistant-> Capability analysis-> Binomial Capability
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