The essential purpose of proficiency testing is to give participants reassurance on the quality of their performance and enable them to identify any trends or errors of consequential magnitude—those likely to affect decisions—in their reported results and then eliminate the causes of those errors. We must recognise, however, that all results of measurements include error. It's only a question of whether the errors are of acceptable size—in effect, whether results are fit for purpose. Fitness for purpose emerges therefore as the key feature of proficiency testing.