Option-3 scheme is a scheme of measurement used in the situations investigating possible changes over the time in longitudinal data. The scheme is planned to prevent measurement outliers causing the unexpected rise in falsely claiming that a change in the data has occurred.

Two measures are taken in the beginning and, if they are closer than a particular threshold, the average of the two is considered to be the estimate of the true mean; or else the third measurement is made, and the mean of closest 'pair' is considered to be the estimate.