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 information for evaluating method performance characteristics when results from interlaboratory method performance studies conforming to internationally recognized protocols are not available. To explore this possibility, results were examined from ongoing proficiency studies of pesticide residue analyses in celery, carrot, and grape purees, and in wine. Statistical performance parameters were calculated from 18 data sets analyzed as unknowns by about 60 analysts for 12 analytes in the 25-1,000 microg/kg range, and from presumably parallel control (spike) analyses conducted by about half of the participants. A surprising finding was that recovery of known, independent control additions by the participant did not correlate with the recoveries determined as unknowns in the exercise. The data suggest that censoring or truncating of control data has occurred. The question of substitution of proficiency data for method performance data cannot be answered until the problem of unbiased reporting of control data is resolved.