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Users can
graphically assess the assumptions of constant error variance
and linearity and probe for outlying cases by examining a
scatter plot of level-1 residuals and predicted values. Using
the same data and model of the previous two sections, we now
plot the level-1 residual against its predicted value.
After the model is run, select
Graph Equations...Level-1 residual vs predicted value
from the File menu, which will give us the following
dialog box.
Note that
the X-axis variable, Pred. val. and Y-axis
variable, Level-1 residuals have been pre-selected.
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 level-1 residuals. Select
All groups on same graph in the Pagination section
to display all the residuals pooled across the level-2 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 level-2 classification variable when examining the level-1
residuals.
 
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Model
based graphs: Level-1 residual vs predicted value