APOE and Cognitive impairment: The estimated event risk (KM estimate) in the pooled ROS/MAP/MARS, NACC, and ADNI populations was higher for APOE4 homozygotes than for heterozygotes and non-carriers, in agreement with previous observations of the increased risk of (an earlier) onset of symptoms of cognitive impairment in APOE4 homozygotes [19]: 38% for APOE4 homozygotes, 23% for heterozygotes, and 16% for non-carriers.