Process Analysis - Process (Machine) Capability Analysis - Computational Approach
Once a process is in control, we can ask the question concerning the process capability. Again, the approach to answering this question is based on "statistical" reasoning, and is actually quite similar to that presented earlier in the context of sampling plans. To return to the piston ring example, given a sample of a particular size, we can estimate the standard deviation of the process, that is, the resultant ring diameters. We can then draw a histogram of the distribution of the piston ring diameters. As we discussed earlier, if the distribution of the diameters is normal, then we can make inferences concerning the proportion of piston rings within specification limits. (For non-normal distributions, see Percentile Method. Let us now review some of the major indices that are commonly used to describe process capability.