Numerous studies suggest that the clinical presentation of Alzheimer's disease (AD) is varied and that AD is thus a heterogeneous disorder. Evidence of this inter-individual variability has had an important impact on the approaches to the cognitive rehabilitation of AD, particularly in the early stages of the disease. Research has shown that, despite variable cognitive profiles, many preserved capabilities are observed in the first stages of AD and that the disease does not affect all cognitive functions globally. These preserved capabilities make it possible to apply cognitive interventions. However, numerous methodological difficulties are encountered in evaluating the effectiveness of these interventions, many of which are related to the heterogeneity of the deficits. The purpose of this article is to discuss this heterogeneity and its impact on methodological characteristics, especially participant selection and research design.
