Figure 1. The “depression ruler”.

The figure shows how the six individual items from the melancholia subscale (HAM-D₆) of the Hamilton Depression Rating Scale relates to the severity of depression. Since the items appear orderly as the severity of illness increases (unidimensionality), the individual item scores can be added to a total score, which is a valid measure for the severity of depression. Modified version of figure 4.2 in Bech P., Clinical Psychometrics, 2012, Wiley-Blackwell, Chichester, West-Sussex, UK.