A test, introduced by Gehan in 1965, as an alternative to the Mantel–Haenszel test for comparing two survivor functions (see hazard rate) using censored data (because, for example, some patients are still living). With two samples, of sizes m and n, the test statistic is W, given by The calculation of N_A>B and N_B>A requires examination of the mn pairs of observations (with one member of each pair from each sample). The quantity N_A>B is the number of pairs in which the member chosen from sample A is certain to have a longer lifetime than the member chosen from sample B.