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149
An Alternative to Analysis of Covariance
Specifying Model A
To specify Model A, draw a path diagram similar to the one on p. 148. The path
diagram is saved as the file Ex09-a.amw.
Results for Model A
There is considerable empirical evidence against Model A:
This is bad news. If we had been able to accept Model A, we could have taken the next
step of repeating the analysis with the regression weight for regressing post_verbal on
treatment fixed at 0. But there is no point in doing that now. We have to start with a
model that we believe is correct in order to use it as the basis for testing a stronger no
treatment effect version of the model.
Searching for a Better Model
Perhaps there is some way of modifying Model A so that it fits the data better. Some
suggestions for suitable modifications can be obtained from modification indices.
Requesting Modification Indices
E From the menus, choose View > Analysis Properties.
E In the Analysis Properties dialog box, click the Output tab.
E Select Modification indices and enter a suitable threshold in the field to its right. For this
example, the threshold will remain at its default value of 4.
Chi-square = 33.215
Degrees of freedom = 3
Probability level = 0.000