Deciding on progression of joint damage in paired films of individual patients: smallest detectable difference or change

Progression of radiological joint damage is usually based on the simultaneous assessment of a series of films from an individual patient (paired'', with or without known sequence). In this setting the degree of progression that can be reliably detected above the measurement error is best determined by the smallest detectable change, and overestimated by the traditionally calculated smallest detectable difference. This knowledge is important for calculation of the proportion of patients showing radiographic progression in clinical trials.