Fay and Wu statistics 

Based on theoretical results of Fu, Fay and Wu developed a new statistic (H) that is sensitive to an excess of new marker alleles at high frequency, which is consistent with hitchhiking but not background selection. What is important about this new test statistic is that it can be used in combination with other test statistics (such as Tajima’s D) to reveal more about the pattern of selection. A significantly negative D is consistent with hitchhiking, but it is consistent with many other processes as well. If H is significant also, then background selection or a recent population expansion can be ruled out. Conversely, if D is significant but not H, other explanations besides hitchhiking gain credibility for the reduced level of genetic variation (as long as the power of the H statistic to detect selection can be shown to be high under the conditions of the empirical study). (Otto 2000)