A nonparametric analysis method for evaluation of survey results. Application to ligand assay survey

A nonparametric analysis method that does not depend on gaussian data distributions is proposed for evaluation of proficiency survey results. The proposed method also provides a mechanism for processing results qualitatively reported as "less than" or "greater than" pre-established limits of quantitation. The evaluation limits for acceptability are calculated to include the central 95% of all test results from all methods. Systematic differences in the levels of analyte concentrations caused by differences in analytical measurement systems are normalized by mathematically transforming the test results from each analytic method by dividing them by their method-specific group median. This nonparametric method was compared with the traditional mean +/- 2 SD limits using data collected for digoxin, free thyroxine, and quantitative chorionic gonadotropin measurement in the College of American Pathologists Ligand Assay Survey. The nonparametric method more accurately classified 5% of the results as "unacceptable." When more than 2.5% of the test results for a method were designated as "less than," no lower limit was used for evaluation and only 2.5% of results were classified as "unacceptably high." The current College of American Pathologists procedure favors analytical methods with larger coefficients of variation by setting wider limits of acceptability, while the proposed procedure favors methods with smaller coefficients of variation.