Example 1: A
2 x 3 Between-Groups Factorial ANOVA Design
This example is based on a fictitious data set presented in Lindeman
(1974). Suppose that you have conducted an experiment to address the nature
vs. nurture question; specifically, you test the performance of different
rats in the "T-maze." The T-maze is a simple maze, and the rat's
task is to learn to run straight to the food placed in a particular location,
without errors. Three strains of rats whose general ability to solve the
T-maze can be described as bright,
mixed, and dull,
were used. From each of these strains you rear 4 animals in a free
(stimulating) environment, and 4 animals in a restricted
environment. The dependent measure is the number of errors made by each
rat while running the T-maze problem.
The data for this study are in the STATISTICA
example data file Rats.sta. Open
this data file via the File - Open Examples
menu; it is in the Datasets folder.
A portion of this file is shown below.
Specifying the Analysis.
Select General Linear Models from the Statistics - Advanced
Linear/Nonlinear Models menu to display the General Linear Models (GLM) Startup Panel
in which you can enter the specifications for the design. Select Factorial ANOVA as the Type
of analysis and Quick specs dialog
as the Specification Method.
Then click the OK button to display
the GLM Factorial ANOVA Quick Specs dialog.
In the data file Rats.sta,
1-free and 2-restricted
were used in the categorical
predictor variable Envirnmt
to denote whether the respective rat belongs to the group of rats that
were raised in the free or restricted environment, respectively. You may
also refer to categorical predictor variables as grouping
variables, coding variables, or between-groups factors. These variables
contain the codes that were used to uniquely identify to which group in
the experiment the respective case belongs.
The codes used for the second categorical predictor variable (Strain) are 1-bright,
2-mixed, and 3-dull.
variable in an experiment is the one that depends on or is affected
by the predictor variables; in this study this would be the variable Errors, which contains the number of
errors made by the respective rat running the maze.
This is a 2 (Environment) by
3 (Strain) between-groups factorial
design. The variables Envirnmt
and Strain are the categorical
predictor variables, and variable Errors
is the dependent variable. Click the Variables
button on the Quick tab, specify these variables
in the Dependent and Categorical
predictor variable lists, and then click the OK
button to return to the GLM Factorial ANOVA Quick Specs dialog.
Next, specify the codes
that were used to uniquely identify the groups; click the Factor
codes button and either enter each of the codes for each variable
or click the All button for each
variable to enter all of the codes for that variable. Then click the OK button to again return to the GLM
Factorial ANOVA Quick Specs dialog.
Now click the OK button to
begin the analysis. When complete, the GLM Results dialog is displayed.
dialog offers a number of output options. For now, click the All
Effects button (located on the Quick tab) to produce a spreadsheet
displaying the summary ANOVA table for the analysis.
Summary ANOVA table. This table
summarizes the main results of the analysis. Note that significant effects
(p<.05) in this table are
highlighted (in red) in this spreadsheet. You can adjust the significance
criterion (for highlighting) by entering the desired alpha level in the
Significance level field on the
Quick tab. Both of the main effects
(Envirnmt and Strain)
are statistically significant (p<.05)
while their 2-way interaction
is not (p>.05).
Reviewing marginal means. The
marginal means for the Envirnmt
main effect will now be reviewed. (Note that the marginal means can be
calculated as unweighted or weighted means, or as least
squares means.) First, click the All
effects/Graphs button to open the Table of All Effects dialog.
In this dialog, select the Envirnmt
main effect and select the Spreadsheet
option button under Display (see
above), then click the OK button
to produce a spreadsheet with the table of marginal means for the selected
The default graph for all spreadsheets with marginal means is the means
plot. In this case, the plot is rather simple. To produce this plot of
the two means for the free and
restricted environment, return
to the Table
of All Effects dialog (by clicking the All
effects/Graphs button on the Quick tab) and change the Display option to Graph,
and again click the OK button.
It appears that rats that were raised in the more restricted
environment made more errors than the rats raised in the free
environment. Now, look at all of the means simultaneously, that is, at
the plot of the interaction
of Environmt by Strain.
Reviewing the interaction plot.
Once again, return to the Table
of All Effects dialog and this time select the interaction
When you click the OK button,
the Arrangement of Factors dialog will be
As you can see, you have full control over the order in which the factors
in the interaction will be plotted. For this example, select STRAIN
under x-axis, upper and ENVIRNMT under Line
pattern (see above), click the OK
button, and the graph of means is then displayed.
The graph below nicely summarizes the results of this study, that is,
the two main effects pattern. The rats raised in the restricted
environment (dashed line) made more errors than those raised in the free environment (solid line). At the
same time, the dull rats made
the most errors, followed by the mixed
rats, and the bright rats made
the fewest number of errors.
Post Hoc Comparisons
of Means. In the previous plot, one might ask whether the mixed strain of rats was significantly
different from the dull and the
bright strain. However, no a priori hypotheses about this question
were specified, therefore, you should use post
hoc comparisons to test the mean differences between strains of
rats (refer to the Introductory
Overview for an explanation of the logic of post
Specifying post hoc tests. After
returning to the GLM
Results dialog, click the More results
dialog to display the larger GLM Results dialog, and then click on
tab. For this example, select the Effect
Strain in order to compare the
(unweighted) marginal means for that effect.
Choosing a test. The different
post hoc tests on this dialog
all "protect" you to some extent against capitalizing on chance
(due to the post hoc nature of
the comparisons). All tests allow you to compare means under the assumption
that you bring no a priori hypotheses
to the study. These tests are discussed in the Post-hoc tab topic. For now, simply
click the Scheffé test button.
This spreadsheet shows the statistical significance of the differences
between all pairs of means. As you can see, only the difference between
group 1 (bright) and group 3
(dull) reaches statistical significance
at the p<.05 level. Thus,
you would conclude that the dull
strain of rats made significantly more errors than the bright
strain of rats, while the mixed
strain of rats is not significantly different from either.
The ANOVA/MANOVA and GLM
Introductory Overview - Assumptions and effects of violating assumptions
topic discusses the assumptions underlying the use of ANOVA techniques.
These same assumptions apply to ANOVA performed using the general linear
model. Now, we will review the data in terms of these assumptions. Return
to the GLM
Results dialog and click on the Assumptions
tab, which offers many different tests and graphs; some are applicable
only to more complex designs.
Distribution of dependent variable.
ANOVA assumes that the distribution of the dependent
variable (within groups) follows the normal
distribution. You can view the distribution for all groups combined,
or for only a selected group by selecting the group in the Effect
drop-down box. For now, select the Environmt*Strain interaction effect and click
the Histograms button under Distribution of variables within groups.
Groups dialog is first displayed in which you can select to view the
distribution for all groups combined, or for only a selected group.
For this example, click the OK
button to accept the default selection of All
Groups, and a histogram of the distribution will be produced.
It appears as if the distribution across groups is multi-modal, that
is to say, it has more than one "peak." You could have anticipated
that, given the fact that strong main effects were found. If you want
to test the homogeneity assumption more thoroughly, you could now look
at the distributions within individual groups, or plot the histograms
of the within-cell residuals (deviations from the within-cell means).
Instead, a potentially more serious violation of the ANOVA assumptions
will be tested.
Correlation between mean and standard
deviation. Deviation from normality is not the major "enemy"
of validity of ANOVA; the most likely "trap" to fall into is
to base one's interpretations of an effect on an "extreme" cell
in the design with much greater than average variability. Put another
way, when the means
and the standard
deviations are correlated
across cells of the design, then the performance (alpha
error rate) of the F-test deteriorates
greatly, and you may reject the null hypothesis with p<.05
when the real p-value is possibly
as high as .50!
Now, look at the correlation between the 6 means and standard deviations
in this design. You can elect to plot the means vs. either the standard
deviations or the variances
by clicking the appropriate button (Plot
means vs. std. deviations, Variances,
respectively) on the Assumptions tab. For this example,
click the Plot means vs. std. deviations
Note that in the illustration above, a linear fit and regression bands
have been added to the plot via the All Options - Plot: Fitting tab
and the All Options - Plot: Regr. Bands tab.
Indeed, the means and standard deviations appear substantially correlated
in this design. If an important decision were riding on this study, one
would be well advised to double-check the significant main effects pattern
by using for example, some nonparametric procedure (see the Nonparametrics
module) that does not depend on raw scores (and variances) but rather
on ranks. In any event, you should view these results with caution.
Homogeneity of variances. Now,
look also at the homogeneity of variance tests. On the Assumptions tab, various tests
are available in the Homogeneity of
variances/covariances group. You may try a univariate test (Cochran C, Hartley, Bartlett) to compute
the standard homogeneity of variances test, or the Levene's
test, but neither will yield statistically significant results.
Shown below is the Levene's Test for
Homogeneity of Variances spreadsheet.
illustrating the major functional aspects of the GLM
module, this analysis has demonstrated how important it is to be able
to graph data easily (e.g., to produce the scatterplot
of means vs. standard deviations). Had you relied on nothing else but
the F-tests of significance and
the standard tests of homogeneity of variances, you would not have caught
the potentially serious violation of assumptions that was detected in
the scatterplot of means vs. standard deviations. As it stands, you would
probably conclude that the effects of environment and genetic factors
(Strain) both seem to have an
(additive) effect on performance in the T-maze. However, the data should
be further analyzed using nonparametric methods to ensure that the statistical
significance (p) values from
the ANOVA are not inflated.
See also GLM - Index.