A Step by Step Guide for SPSS and Exercise Studies

124 Statistical tests
Dialog box 85
analysis. S.E. of mean predictions provides an estimate of the standard error of
the mean predicted value. A number of different residuals can be saved. For a
description of each residual type, see the Save option under General Linear
Model/Univariate in the Analyze menu. Predicted values can be plotted against
the residuals as shown for Univariate tests. You can also use the Plots option
(see below) to create similar plots.
The Save option calculates three distance measures which can be used to
identify influential cases among the independent variables. Mahalanobis
distance is a measure of how much the value of a case differs in the
independent variables from the average of all other cases. Large Mahalanobis
distances signify potential outlier cases. This measure is distributed as a chisquare
with degrees of freedom equal to the number of predictors. In this
example, the predictors are two. Looking at the chi-square distribution table in
the appendices of any statistical book, you will find that the critical value of the
chi-square with 3 degrees of freedom at the p ˆ .01 level is 9.21. Therefore,
cases with Mahalanobis distance above 9.21 are potential outliers. Note that
Tabachnick and Fidell (1996) suggest that this test can be used to identify