Relationship of (known) control values to (unknown) test values in proficiency studies of pesticide residues

Proficiency studies have been suggested as an alternative source of informa tion for evaluating method performance characteristics when results from in terlaboratory method performance studies conforming to internationally reco gnized protocols are not available. To explore this possibility, results we re examined from ongoing proficiency studies of pesticide residue analyses in celery, carrot, and grape purees, and in wine. Statistical performance p arameters were calculated from 18 data sets analyzed as unknowns by about 6 0 analysts for 12 analytes in the 25-1000 mu g/kg range, and from presumabl y parallel control (spike) analyses conducted by about half of the particip ants. A surprising finding was that recovery of known, independent control additions by the participant did not correlate with the recoveries determin ed as unknowns in the exercise. The data suggest that censoring or truncati ng of control data has occurred. The question of substitution of proficienc y data for method performance data cannot be answered until the problem of unbiased reporting of control data is resolved.