If the diagnosis of AD is combined with the APOE ε4 gene test and psychiatric scale, assuming that the physician estimates the probability of AD based on the patient's history and physical signs and the results of the scale test is 50% (the diagnostic accuracy of NINCDS-ADRDA is reported to be 65%-96% [25]), then, the posterior probability is 79%. Here, APOE is linked to Alzheimer disease.