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Commonly used a priori contrasts are available to perform hypothesis
testing. Additionally, after an overall F test has shown significance,
you can use post hoc tests to evaluate differences among specific
means. Estimated marginal means give estimates of predicted mean
values for the cells in the model, and profile plots (interaction plots)
of these means allow you to easily visualize some of the relationships.
Residuals, predicted values, Cook's distance, and leverage values can
be saved as new variables in your data file for checking assumptions.
WLS Weight allows you to specify a variable used to give
observations different weights for a weighted least-squares (WLS)
analysis, perhaps to compensate for a different precision of
measurement.
Example.Data are gathered for individual runners in the Chicago
marathon for several years. The time in which each runner finishes is
the dependent variable. Other factors include weather (cold, pleasant,
or hot), number of months of training, number of previous marathons,
and gender. Age is considered a covariate. You might find that gender
is a significant effect and that the interaction of gender with weather
is significant.
Methods.Type I, Type II, Type III, and Type IV sums of squares can
be used to evaluate different hypotheses. Type III is the default.
Statistics.Post hoc range tests and multiple comparisons: least
significant difference, Bonferroni, Sidak, Scheffé, Ryan-Einot-
Gabriel-Welsch multiple F, Ryan-Einot-Gabriel-Welsch multiple
range, Student-Newman-Keuls, Tukey's honestly significant
difference, Tukey's b, Duncan, Hochberg's GT2, Gabriel, Waller-
Duncan t test, Dunnett (one-sided and two-sided), Tamhane's T2,
Dunnett's T3, Games-Howell, and Dunnett's C. Descriptive statistics:
observed means, standard deviations, and counts for all of the
dependent variables in all cells. The Levene test for homogeneity of
variance.
Plots.Spread-versus-level, residual, and profile (interaction).