
 EPA  DistributionFree Methods 
  Before computers, the mathematically "nice" features of the Gaussian, or normal, distribution meant that parametric methods were used most often. 
  Many of the classical tests that require an assumption of normality have nonparametric counterparts where the actual observed values are replaced by their ranks. 
  Nonparametric methods should be used when the data are known to not be normally distributed, for example, when the distribution of the data has a long tail or ordinal values (e.g., low, medium, or high) rather than continuous values (e.g., 0100) with a unimodal distribution. 
 www.epa.gov /bioindicators/statprimer/distribution.html (249 words) 
