Hi. Since the modification of the power.prop.test() function in R 3.1.3 ("power.t.test() and power.prop.test() now make use of the extendInt option of uniroot() and hence work in more extreme cases. (PR#15792)") the function can display proportion over 1 without warning message. Example: power.prop.test(n=30, p1=0.90, p2=NULL, sig.level=0.05, power=0.8, alternative="two.sided", strict=TRUE) Two-sample comparison of proportions power calculation n = 30 p1 = 0.9 p2 = 1.030182 sig.level = 0.05 power = 0.8 alternative = two.sided NOTE: n is number in *each* group This was not the case before the modification. Moreover, in ?power.prop.test(), the following sentence is not clear: "If one of them is computed p1 < p2 will hold, although this is not enforced when both are specified". Many thanks for your help.