Mahieux et al 1 report on an interesting study of predictors of dementia in Parkinson’s disease. We have certain reservations about the results because of the methodological limitations of the study.

The diagnosis of dementia was made by retrospective case note review or retrospective completion of questionnaires including some by non-specialists which raises questions about the validity of diagnosis. The researchers making the diagnosis of dementia were not blind to neuropsychological test score, which raises the possibility of observer bias. The study was uncontrolled so that the estimate of the incidence of dementia cannot be adequately interpreted because it is not known what the incidence in the general population would be using this method of case identification.

The findings of predictors of dementia are a little puzzling. Aithough four variables are …