The differences between control limits and spec (specification) limits

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The differences between control limits and spec (specification) limits may seem irrelevant or nonexistent to those outside process production, but the gulf between them is in fact huge. In fact, they are two entirely different animals. Spec limits may be designated by a customer, engineer, etc., indicating the allowable spread of a given measurement. Control … Continued

Gage R&R study questions answered

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We frequently entertain questions about MSA and specifically, gage R&R. Below are two questions we recently received: Question #1: “What are the requirements for the parts chosen in a study? Do the parts have to have the same specification?” Answer: The parts selected should be representative of the process variation that is producing them. This implies that … Continued

The difference between run charts and control charts

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A customer recently asked one of our support representatives the following questions: What is the difference between a run chart and a control chart? And when should I use one vs. the other? These are great questions because they allow us to highlight some of the benefits of control charts. When you create any chart, … Continued

Stats tip: Within and between subgroup variation clarified

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I received a question after my last blog post asking me to clarify the concept of within and between subgroup variation which is used in calculating Cpk, Cp, Cr, Ppk, Pp, Pr and other statistics. Here is an example I used to help explain the differences. Let’s say that every day I run about 30 … Continued

Stats tip: Help! My Ppk is larger than my Cpk

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I often get asked SPC application questions such as: Can Ppk be larger than Cpk? Can Cpk be larger than Cp? Do you recommend CPM, Cpk, Ppk or something else? I prefer to answer these process capability questions with simple one-word answers, but that doesn’t usually satisfy the quality zealot’s curiosity. So here’s the long … Continued