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Statistical Advisor, Test Hypotheses Regarding Differences between Groups or Variables

The distinction between:

  • comparison of variances in groups or variables
  • general nonparametric comparison or comparison of survival time between groups

should be based on the following considerations.

  1. Is the variable (or variables) on which the comparison is to be based measured on at least an interval scale level? Interval scale measurement means that, for example, a difference between the values 1 and 2 in that variable is comparable to the difference between 3 and 4. Often, the values of variables contain only rank order information, and do not allow us to compare intervals. Refer to the manual (Elementary Concepts) for more details concerning this issue.
  2. Does the distribution of data points follow (approximately) the normal distribution? You can easily check this condition by viewing a histogram.
  3. Is the data set reasonably large? For example, are there at least 20 observations in the comparison?

If any of these three conditions are not met, it is advisable to verify any results via the appropriate nonparametric tests discussed in NONPARAMETRICS & DISTRIBUTIONS.