Bivariate/multivariate data cleaning can also be
important (Tabachnick & Fidell, p 139) in multiple
regression. Most regression or multivariate statistics
texts (e.g., Pedhazur, 1997; Tabachnick & Fidell,
2000) discuss the examination of standardized or
studentized residuals, or indices of leverage.
Analyses by Osborne (2001) show that removal of
univariate and bivariate outliers can reduce the
probability of Type I and Type II errors, and improve
accuracy of estimates.
Outlier (univariate or bivariate) removal is
straightforward in most statistical software.
However, it is not always desirable to remove
outliers. In this case transformations (e.g., square
root, log, or inverse), can improve normality, but
complicate the interpretation of the results, and
should be used deliberately and in an informed
manner. A full treatment of transformations is
beyond the scope of this article, but is discussed in
many popular statistical textbooks.