This Assessment News article examines the role of measurement error in standardised tests and what it means for progress measures. Charles Darr explains that standardised test scores and the progress trajectories we infer from them do not necessarily reflect a student's true achievement level. Measurement error is the normal day-to-day variation in test performance from factors such as fatigue, anxiety, interruptions, or guessing, and it can make apparent trends (rises, dips, widening gaps) look meaningful when they are not. Teachers are shown what to look for to interpret both single scores and progress measures more accurately.