A Tutorial on Estimating the Precision of Individual Test Scores

When using tests to assess individuals, precision of individual test scores is of great importance. Although it is generally known that different test scores are measured with varying precision, traditionally, measurement precision is quantified using a single value known as the standard error of measurement (SEM). In the practice of testing, the SEM is used as a one-size-fits-all measure for each test score. This practice emphasizes the need for a score-related precision estimate that shows which scores are precise and which scores lack precision. We discuss several score-related precision estimates based on classical test theory and item response theory, and provide open-source statistical software included in JASP that enables computation of these estimates. Using score-related precision estimates, decisions based on test scores are expected to show less bias than the common score-unrelated SEM.