Filter
Top Products
255
Sörbom’s Alternative to Analysis of Covariance
intercept for the experimental group is significantly different from the intercept for the
control group (which is fixed at 0).
Model E
Another way of testing the difference in post_verbal intercepts for significance is to
repeat the Model D analysis with the additional constraint that the intercept be equal
across groups. Since the intercept for the control group is already fixed at 0, we need
add only the requirement that the intercept be 0 in the experimental group as well. This
restriction is used in Model E.
The path diagrams for Model E are just like that for Model D, except that the
intercept in the regression of post_verbal on pre_verbal is fixed at 0 in both groups.
The path diagrams are not reproduced here. They can be found in Ex16-e.amw.
Results for Model E
Model E has to be rejected.
Comparing Model E against Model D yields a chi-square value of 51.018 (= 55.094 –
3.976) with 1 (= 6 – 5) degree of freedom. Model E has to be rejected in favor of Model
D. Because the fit of Model E is significantly worse than that of Model D, the
hypothesis of equal intercepts again has to be rejected. In other words, the control and
experimental groups differ at the time of the posttest in a way that cannot be accounted
for by differences that existed at the time of the pretest.
This concludes Sörbom’s (1978) analysis of the Olsson data.
Fitting Models A Through E in a Single Analysis
The example file Ex16-a2e.amw fits all five models (A through E) in a single analysis.
The procedure for fitting multiple models in a single analysis was shown in detail in
Example 6.
Chi-square = 55.094
Degrees of freedom = 6
Probability level = 0.000