Big errors and little errors

In clinical assay evaluations, most of the time, focus is on “little” errors. What I mean by little errors are average bias and imprecision that exceed goals. Now I don’t mean to be pejorative about little errors since if bias or imprecision don’t meet goals, the assay is unsuitable. One of the reasons to distinguish between big and little errors is that often in evaluations, big errors are discarded as outliers. This is especially true in proficiency surveys but even for a simple method comparison, one is justified in discarding an outlier because the value would otherwise perturb the bias and imprecision estimates.

But big errors cause big problems and most evaluations focus on little errors, so how are big errors studied? Other than running thousands of samples, a valuable technique is to perform a FMEA (Failure Mode Effects Analysis). This can or should cover user error, software, interferences, besides the usual items. A FMEA study is often not very enthusiastically received but it is a necessary step in trying to ensure that an assay is free from both big and little errors. Of course, even with a completed FMEA, there are no guarantees.

 

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