Psychometric testing is used in various contexts and for different clienteles. Tests serve to characterize the “personality” of an individual by means of scores on a few descriptive scales, to estimate a candidate’s aptitude for some foreseen task or job output, to uncover someone’s tendency for suicide or for the commission of a crime, etc. The tests, generally based on one or a few measurement scales, are technically defined by their reliability, their validity and their norms of interpretation. This article deals with the determination of the tests’ norms, specifically with the accuracy of the psychometric thresholds, or percentiles, and, as a corollary, with the validity of the ensuing decision based on these norms.