Estimation of imprecision in immunoassay quality assessment programmes.

A computer was used to produce and store a large body of simulated thyroxine and thyrotropin radioimmunoassay results over wide concentration ranges. Sets of 24 results, designed to represent results returned to an external quality assessment survey, were randomly drawn from the stored results in order to compare limited data survey estimates of imprecision with the 'true' values obtained from the population data. Results show that estimation of an imprecision profile has advantages over the usual index of imprecision, but that neither form of estimate is particularly reliable when the available data is limited. Results suggest that survey organisers should place considerably less emphasis on results from any one survey and greater emphasis on analysing cumulative data and method comparison data.