Stratified data: a permutation approach for hypotheses testing.

The present work aims at presenting a general nonparametric alternative to the well known van Elteren test for two-sample stratified analysis. We developed a solution based on permutation tests that considers the Nonparametric Combination (NPC) methodology for reducing the dimensionality of the problem. A simulation study to compare performances of proposed test with those of the usual van Elteren test and of aligned rank test has been performed considering both continuous and ordinal data. Results shows the respect of nominal α-level under H0 even for small sample sizes. A real application example is also presented.