Criteria are given for the choice of scale prior to estimation of repeatability. Recommendations of Bland and Altman should then be used for expressing repeatability and agreement of methods of measurement on the same scale. Repeatability of measurements on different scales should be compared using the appropriate ratio of variances, or intraclass correlation coefficient. A reference range for diagnosis requires a high ratio of between-subject variation to total variation. The index of separation between diseased and healthy subjects should be used whenever possible. Changes within patients should be compared with reference change ranges, and not against the diagnostic range.