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Dear Drs. Muthen,
I am running path analyses and comparing the AIC values of competing models. Since some of my variables are categorical and some continuous, sometimes a change in only the direction of one of the direct effects may change the model from linear regression to logistic regression. I got the impression that in all these cases, the AIC value significantly increases, independent on the specific variables and the relationships among them. Am I correct? If so, what is the right way to compare between these models?
Thanks in advance, Hadas |
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| I don't understand your question - perhaps you reformulate it. |
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I am sorry that I was not clear.
I will give you an example.
I wish to compare the AIC values of two alternative models, with parameters A-D, where “A” is a continuous variable and “B” is a categorical variable. The models are identical except for the last line:
MODEL 1:
A on C;
A on D ;
B on A;
AIC= 301.750
MODEL 2:
A on C;
A on D ;
A on B;
AIC= 241.887
As you see, I received a lower AIC value for the second model and I was wondering whether the reason for the lower AIC value in model 2 could be that in this model the last line represents a linear regression whereas in model 1 it represents a logistic regression (since B is categorical).
Thanks in advance, Hadas |
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| The two AIC values are in different metric. You have to have the same DV's for two models to have the same logL and therefore AIC metric. Your two models have different sets of DVs due to variable B. |
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Thanks for your answer. Does that mean that I have to have also the same mediators in two models if I wish to compare between them, using AIC?
e.g., I cannot compare between the models:
Model 1:
A on C;
D on A;
Model 2:
D on B;
B on G;
Thanks, Hadas |
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| Yes, mediators are dependent variables. To compare, the variables on the left-hand side of ON must be the same. |
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Thank you very much for the helpful answer. Is there any way to compare between competing models that have different mediators? For example, if I wish to know whether the indirect relations between A and B are mediated by C or D?
Hadas |
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| Not that I know of when the dependent variables are not the same. |
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