The Cpkindex is a common process performance standard which quantifies process location and variation. However, the index is also frequently used to estimate the proportion of non-conforming items without regard to the underlying restrictions or assumptions that apply. An example based on an actual industrial case study will serve to underscore the consequences of the indiscriminate use of the index for this purpose. Subsequently, additional experimentation and analyses will be conducted to explore the generality of the results obtained from the case study