The role of the observer in the reliability of the Dutch Discomfort Scale-Dementia of Alzheimer Type (DS-DAT). The Discomfort Scale of Dementia of the Alzheimer Type (DS-DAT) is an instrument to assess discomfort in severely demented patients. No data on the reliability of assessment using a Dutch translation were available. In this paper, we analyse the role of the observer in the reliability of rating. This is of importance for studies in which many physicians perform multiple assessments. Twenty-eight nursing home physicians in training rated the DS-DAT in five nursing home patients with dementia presented on videotape. This was repeated after five months. All the physicians were previously trained in the use of the instrument. The results were statistically analysed using random effects analysis of variance. The Intra-class Correlation Coefficient (ICC) was 0.74 for inter-observer reliability and 0.97 for intra-observer reliability. Variance between subsequent assessments was small, but physicians appeared to differ somewhat among themselves in the way they rated the videotaped patients. A future complete reliability assessment of rating the DS-DAT in clinical practice would involve patient variation as well, scoring patients in clinical practice.