The push to develop treatments for Alzheimer's disease has yielded a greater understanding of the disease, but has failed to generate successful new drugs. To blame are the many undefined subtypes of mild cognitive impairment, a precursor to Alzheimer's disease. But if scientists grouped people with similar types of cognitive impairment, they could more precisely test the impact of investigational drugs.
https://www.sciencedaily.com/releases/2017/07/170728092555.htm