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Example 9
Drawing a Path Diagram for Model C
To draw the path diagram for Model C:
E Start with the path diagram for Model B.
E Right-click the arrow that points from treatment to post_verbal and choose Object
Properties
from the pop-up menu.
E In the Object Properties dialog box, click the Parameters tab and type 0 in the
Regression weight text box.
The path diagram for Model C is saved in the file Ex09-c.amw.
Results for Model C
Model C has to be rejected at any conventional significance level.
If you assume that Model B is correct and that only the correctness of Model C is in
doubt, then a better test of Model C can be obtained as follows: In going from Model
B to Model C, the chi-square statistic increased by 52.712 (that is, ),
while the number of degrees of freedom increased by 1 (that is, 3 – 2). If Model C is
correct, 52.712 is an observation on a random variable that has an approximate
chi-square distribution with one degree of freedom. The probability of such a random
variable exceeding 52.712 is exceedingly small. Thus, Model C is rejected in favor of
Model B. Treatment has a significant effect on post_verbal.
Fitting All Models At Once
The example file Ex09-all.amw fits all three models (A through C) in a single analysis.
The procedure for fitting multiple models in a single analysis was demonstrated in
Example 6.
Chi-square = 55.396
Degrees of freedom = 3
Probability level = 0.000
55.396 2.684–