Prevalence of Alzheimer's Disease

To the Editor.— The study by Evans et al 1 on Alzheimer's disease includes a reference to a previous publication by Rocca et al. 2 Evans et al refer to that study as estimating Alzheimer's disease at a prevalence of 11.2% at age 66 years and older. They cite this article as being consistent with their finding of 10.3%, which was surprisingly high. I was not able to locate this finding of 11.2% prevalence anywhere in the Rocca et al article. In fact, Rocca et al plainly state in their abstract, as well as in the text, that the prevalence rate at this age ranges between 1.9% and 5.8%. The finding of 11.2% only appears in Table 2, where it seems to reflect the proportion of the general population that is older than 65 years. Considering the unexpected and important findings in the East Boston study, and since the Rocca