3. Descriptive Validity
Summaries of the Data
Any evaluation requires summaries of the data, with some summaries more useful than others. Indeed, some summaries can be misleading. Perhaps the most well known example is the manner in which outliers can affect measures of central tendency: the mean, median, or mode. The mean is especially vulnerable. But outliers can dramatically affect many other summary statistics such as the standard deviation, the Pearson correlation coefficient, and the regression coefficient.