Comments on the paper by Möller et al. (1989): problems in single-case evaluation.

Möller et al. (1989) investigated the effects of sleep deprivation on depressive patients using a single-case analysis based on Kleiter's 1986 approach. Here it is discussed why it is so difficult to perform single-case analyses and why Kleiter's approach in particular may lead to wrong interpretations of the data. It is indicated that any kind of single-case analysis is based on assumptions which possibly do not hold and that it is therefore always precarious to base substantial conclusions on this type of analysis.