For some problems involving a parameter of interest and a nuisance parameter, it is possible to define a statistic sufficient for the parameter of interest. The definition has a number of applications in nonparametric theory. Two theorems are derived and used by way of illustration to prove that the sign test is a uniformly most powerful test for the nonparametric form of the single sample problem of location.
"Sufficient Statistics with Nuisance Parameters." Ann. Math. Statist. 27 (3) 838 - 842, September, 1956. https://doi.org/10.1214/aoms/1177728191