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John J. Hickey

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A Simple Model of a Variance Stable Process

Most fairly accurate descriptions of equipment and/or process lifetimes assume that failure rates follow a three period I II III “bathtub-curve pattern” where failures/errors: I – Decrease during the debugging or improvement time period. II – Remain relatively constant and at their lowest levels during the normal equipment or process operating period. III – Increase…


Using Efficient Process Tolerance and Mean Shift Tests

The sequential probability ratio test, or SPRT, can be used as an efficient tool for process tolerance and mean shift determinations. It also provides for simplifying insights into the nature of random mean shifts when process tolerances and Type I/II errors are selected. The cumulative sum design of the test naturally compensates for random errors,…