Clinicopathological studies of the dementias from an epidemiological viewpoint.

Studies which examine patients with and without dementia during life and then examine brain tissue after death are extremely difficult to conduct. There are now several such studies published, representing a major contribution to the understanding of the pathologies of the dementias. These studies were not designed to represent population samples and, from an epidemiological viewpoint, they are flawed because none are population-based and none represent the full range of function observed during life. It is therefore important to examine the available studies for their contributions and biases.