Hi there, I'd be very grateful for help with a problem. I have some independent or predictor variables, and I am testing for a statistically significant relationship with various dependent or outcome variables. Some of the predictor variables are dominant - I know they have a strong influence on results. So when some of the other predictor variables are found to have a statistically significant correlation, I don't know if that is simply because they are in turn associated with the dominant variable. That is, are all the observed effects explained by the dominant variable? So I am trying to control for the influence of the dominant variables by using multinomial logistic regression and testing for interaction effects.
The variables are mostly nominal categorical variables or dichotomous Yes/No variables. This has made it harder to find a method and get help, since most guidebooks assume that users have continuous variables.
I ran the exercise in SPSS via Analyse > Regression > Multinomial Logistic. I only used two predictor variables to test. Before running the model, I tried to add for interaction effects by going to Model then selecting Custom/Stepwise and adding either 'Interaction' or 'All 2-way' for selected variables.
However, when I add interaction terms, I don't get a result in the Likelihoods Ratio box for several of the variables. (I read that the Likelihoods Ratio is what I should look at to tell me if my nominal/categorical variables have a statistically significant relationship with the outcome variable.) Instead, SPSS gives a message saying 'This reduced model is equivalent to the final model because omitting the effect does not increase the degrees of freedom'.
When I don't specify interaction effects, I do get results in the Likelihoods Ratio box for both variables. But this doesn't help me; I want to test for interaction, like I said.
I looked online for why I am getting this message, and one forum suggests it is because the variables are limited. But the variables that I tested are fairly simple and meaningful variables: one is a simple Yes/No dichotomous variable and the other has 3 categorical options.
Does anyone know why SPSS is doing this? Or if there is a better way to 'control' for the effect of dominant predictor variables so I can see if any other variables are having an effect?
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Rebecca
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