Transforming the data to be approximately well modeled by a Normal distribution.When sampling from stable, but non-normal distributions, other strategies to obtain meaningful capability estimates may be appropriate, including: As such, the use of confidence intervals for the true capability values may also be reported. Others, however, note that the capability estimates are themselves merely statistics, or point estimates of the true capability of a process. Many quality practitioners report solely the numerical values of the capability estimates. For example, C p and C pk estimates are highly sensitive to the assumption that one is sampling from a normal distribution-that is, most of the data points are concentrated around the average (mean), forming a bell–shaped curve.įurthermore, sampling from a stable system is essential to obtaining meaningful estimates of process performance for future production. There are both positive and negative aspects to capability estimates. Practical Concerns when Conducting Capability Studies It is as close to pure capability as you’re likely to get. Because the pooled, within-group standard deviation is calculated on observations taken close together in time, there is no opportunity for it to be contaminated by assignable sources of variation. Estimates of the process capability made this way would be representative and independent of process mean changes that might take place from one sampling time to the next. Table 1: Assessing Process Capability: Sampling Scheme Exampleįor greater assurance yet, you might want to include several production runs with perhaps fewer sampling times per production run. You would pool the eight individual standard deviations yielding a thickness capability estimate based on (8 X (30 - 1)) = 232 degrees of freedom. Better yet, you might assure representation by taking those 30 consecutive tablets repeatedly over eight time periods spaced evenly throughout a production run (Table 1). If you’re interested in tablet thickness, you might want to base your estimate of process capability on the standard deviation calculated from 30 consecutive tablets. The measurement of process capability is more complicated than that.įor example, suppose you have a rotary tablet press that produces 30 tablets, one from each of 30 pockets per rotation. Some textbooks teach users to wait until the process reaches equilibrium, take roughly 30 samples and calculate their standard deviation however, it is difficult to know when the process reaches a state of equilibrium and if the recommended samples are representative of the process. Assessing Process CapabilityĪssessing process capability is not easy. Typically this involves calculating some ratio of the specification limits to process spread.
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