What is randomization?

Randomization requires two criteria:

  1. All elements within a population have an equal chance of selection.
  2. No single element can be selected more than once.
A good example of proper randomization for a national sample see:
Laumann, E. O., Michael, R. T.,  Gagnon, J. H.  & Michaels, S. (1994). The Social Organization of Sexuality: Sexual Practices in the United States Chicago: University of Chicago Press.

A good example of improper randomization for a national sample see:
Hite, S. (1987). Women and Love: A Cultural Revolution in Progress. New York: Alfred A. Knopf.