Variables Are Normally Distributed.
Regression assumes that variables have normal
distributions. Non-normally distributed variables
(highly skewed or kurtotic variables, or variables
with substantial outliers) can distort relationships and
significance tests. There are several pieces of
information that are useful to the researcher in testing
this assumption: visual inspection of data plots,
skew, kurtosis, and P-P plots give researchers
information about normality, and KolmogorovSmirnov
tests provide inferential statistics on
normality. Outliers can be identified either through
visual inspection of histograms or frequency
distributions, or by converting data to z-scores