Users can
graphically assess the assumptions of constant error variance
and linearity and probe for outlying cases by examining a
scatter plot of level1 residuals and predicted values. Using
the same data and model of the previous two sections, we now
plot the level1 residual against its predicted value.
After the model is run, select
Graph Equations...Level1 residual vs predicted value
from the File menu, which will give us the following
dialog box.
Note that
the Xaxis variable, Pred. val. and Yaxis
variable, Level1 residuals have been preselected.
Select All groups (n=22)
in the Number of groups to include all the 22 children
in the display. Click the selection button for Scatter
plot in the Type of plot section to request a scatter
plot of the predicted values by level1 residuals. Select
All groups on same graph in the Pagination section
to display all the residuals pooled across the level2 units.
To examine the residuals for individual children, choose either
of the other pagination options. Click OK.
The plot
suggests that there is a tendency for the residual scatter
to get narrower at the smallest predicted values and to get
wider around the interval between 150 and 170. The residuals
seem to follow a slightly curvilinear trend as well.
They may suggest that there is a specification error in the
model.
Users can also choose to include
a level2 classification variable when examining the level1
residuals.
