Fay and Wu's H is a statistical test created by and named after two researchers Justin Fay and Chung-I Wu.[1] The purpose of the test is to distinguish between a DNA sequence evolving randomly ("neutrally") and one evolving under positive selection. This test is an advancement over Tajima's D,[2] which is used to differentiate neutrally evolving sequences from those evolving non-randomly (through directional selection or balancing selection, demographic expansion or contraction or genetic hitchhiking). Fay and Wu's H is frequently used to identify sequences which have experienced selective sweeps in their evolutionary history.