The figure below shows a hypothetical retrospective analysis that compares cognitive performance in two groups of individuals a) persons that have AD and b) persons that are dementia free. In this type of analysis, the two groups are examined retrospective to compare performance on cognitive task, for example, three years before the diagnosis. Average group performance can then be compared to see whether persons that develop AD have significantly lower scores on a memory task than healthy controls. Although retrospective analysis can help to identify patterns of cognitive impairment that occur in the preclinical phase of AD, they only provide information on a group level. In other words, on average the preclinical AD group perform worse than normal controls, but there can be individual variation. If you look at the figure carefully you can see that although most persons that later develop AD have low performance on the memory task, there are a number of individuals that still score quite well. Similarly, although most persons in the nondemented group perform well on the task, there are some individuals that score at low levels, even though they do not later develop dementia. What is then important is to verify whether a certain test (or classification such as MCI) can be used to positively predict conversion to dementia. One question, for example, could be ?How many persons with a score of XX on an episodic memory task will progress to dementia over one year?? Thus, prospective studies are needed to identify tests that can identify individuals in the preclinical stage of AD.

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