(This article was first published on SAS and R, and kindly contributed to R-bloggers)
Permutation tests (section 2.4.3) are a form of resampling based inference that can be used to compare two groups. A simple univariate two-group permutation test requires that the group labels for the observations are exchangeable under the null hypothesis of equal distributions, but allows relaxation of specific distributional assumptions required by parametric procedures such as the t-test (To leave a comment for the author, please follow the link and comment on his blog: SAS and R.
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Zero Inflated Models and Generalized Linear Mixed Models with R.
Zuur, Saveliev, Ieno (2012).