A scheme of measurement used in situations investigating possible changes over time in longitudinal data. The scheme is designed to prevent measurement outliers causing an unexpected increase in falsely claiming that a change in the data has occurred. Two measures are taken initially and, if they are closer than a specified threshold, the average of the two is considered to be an estimate of the true mean; otherwise a third measurement is made, and the mean of the closest ‘pair’ is considered to be the estimate.