Using Frequency of Occurrence to Find Root Cause


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The frequency of occurrence of an unexpected characteristic or non-conformance is an important indication of possible root cause. When you tie frequency of occurrence numbers back to the process, you can gain some insight into what may be the root cause.

“Occasionally, I get one of these parts with the drill going off-center.” While this sentence names the problem, it doesn’t give us much insight as to where to look—the drill, I guess.

“I have about half a percent of the parts exhibiting an off-center drill feature on my six spindle.” This gives us enough information to think that perhaps the first couple of inches of one bar on a stock up was crooked, resulting in the off-center drill.

If you can do the math, you can uncover the root cause.


Converting the count to a percentage of production can help you prioritize where in your process to look for the likely cause.

This blog post was originally published on PMPA Speaking of Precision.