EP9 – “Bias estimation” should be called “average bias estimation”

words2EP9, the CLSI standard about method comparison is being revised. What struck me is that the title of the current version includes the term “bias estimation”. This is not accurate. What EP9 estimates is average bias.

Average bias is a useful thing to estimate. For example, it’s good to know that the Beckman Coulter PSA assay is 22% higher on average than any WHO calibrated PSA assay. However, using the word bias instead of average bias implies that any and all biases will be estimated. This is not the case for EP9 since a result with a high bias that is detached from the other results will be excluded from the analysis. Moreover, two assays with the same regression coefficients but with different scatter will exhibit the same average bias (although the confidence intervals for average bias will be different).

Historically, EP9 was released as a Proposed document in 1986; however, it is based on an earlier CLSI (then called NCCLS) document called PSEP-4 released in 1979. The original title was “Comparison of Methods Experiment.” So later versions included the term bias estimation in the title. Note that there are two documents that estimate bias due to interference (EP7 and EP14) and it is only relatively recent that two documents estimate all bias in an assay – EP21 (2003) and EP27 (not yet released). See the CLSI site for details about individual documents.


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