STATISTICS
Precision studies: common mistakes, limitations and points of caution
In this third article in his series on statistics, Stephen MacDonald moves on to look at precision studies, focusing on issues such as their practical use, terms used, study design, and interpretation. Also included is a worked example and important take-home points for the reader.
Precision studies are among the most familiar statistical exercises in laboratory medicine, but they are also among the easiest to oversimplify. The result is often reduced to a single coefficient of variation, usually reported as ‘the CV’. That is a problem, the CV is the final summary of a precision study, not the statistical method itself.
The more useful question is: where
did the variation come from? A well- designed precision study uses repeated measurements to separate variation into components. Some variation occurs between replicate measurements within the same analytical run. Some arise between runs, days, reagent lots, operators, analysers or sites, depending on how the study was designed. The final precision estimate only makes sense when those layers of variation are understood. There is also a terminology point
worth making at the outset. Precision describes closeness of agreement between repeated measurements. The results of a precision study are usually expressed as measures of imprecision, such as the standard deviation (SD) or coefficient of variation (CV). Lower imprecision means beter precision. This article uses ‘precision study’ for the design and ‘imprecision’ for the numerical output. Precision should not be confused
with accuracy. Precision describes scater between repeated measurements; accuracy concerns closeness to the true or accepted value. A method may be precise but biased, imprecise but unbiased
A well-designed precision study uses repeated measurements to separate variation into components.
on average, or poor in both respects. This article focuses on the random component: how to design, analyse and interpret studies of imprecision in routine laboratory practice.
Why we need to understand precision studies Precision studies are not performed simply to satisfy an accreditation
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or verification requirement. They provide evidence about how much random variation can be expected when a measurement procedure is used in practice. That evidence matters when a laboratory introduces a new method, verifies manufacturer claims, modifies an existing procedure, investigates unexpected variability, or assesses whether results are
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