Here we take a serious look at some nonsensical ideas about capability ratios. Following a quick review of predictability and capability and a brief discussion of the traditional ways of characterizing capability and performance, we will consider the shortcomings of four bits of capability confusion found in some of today’s software.

The predictable operation of a process is an achievement rather than being an intrinsic property of that process. This means that the notion of a process mean, a process standard deviation, or a process capability is not well-defined until that process is operated predictably. Given data from a predictable process, we can use statistics for location and dispersion, along with specification values, to estimate the process mean, the process standard deviation, and the process capability in the manner outlined in figure 1.

Figure 1: Estimating parameters for a predictable process

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