A read of the fine print of most statistical tests reveals the assumption that the data is independently and identically distributed. In many fields, this is generally just a different way of restating the assumption that the data is a simple random sample. Where this assumption is not satisfied there are alternatives, provided that the data can be viewed as being something of a probability sample, but they are technical and not available in most programs used for survey analysis; for more information on this point, see:
- White, H. (2001). Asymptotic Theory for Econometricians, Revised Edition. San Diego, Academic Press.
- Cochran, W. G. (1977). Sampling Techniques, Third Edition. New York, Wiley.
- Wooldridge, J. M. (2001). Econometric Analysis of Cross Section and Panel Data, MIT Press.