Sample size for a Mann-Whitney test — size.test.mann • statpsych

Computes the sample size in each group (assuming equal sample sizes) required for the Mann-Whitney test with desired power. A planning value of the Mann-Whitney parameter is required. In a 2-group experiment, this parameter is the proportion of members in the population with scores that would be larger under treatment 1 than treatment 2. In a 2-group nonexperiment where participants are sampled from two subpopulations of sizes N1 and N2, the parameter is the proportion of all N1 x N2 pairs in which a member from subpopulation 1 has a larger score than a member from subpopulation 2.

size.test.mann(alpha, pow, p)

Arguments

alpha: alpha level for hypothesis test
pow: desired power
p: planning value of Mann-Whitney parameter

Value

Returns the required sample size for each group

References

Noether GE (1987). “Sample size determination for some common nonparametric tests.” Journal of the American Statistical Association, 82(398), 645--647. ISSN 0162-1459, doi:10.1080/01621459.1987.10478478 .

Examples

size.test.mann(.05, .90, .3)
#>  Sample size per group
#>                     44

# Should return:
# Sample size per group
#                    44