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| It is suggested that any continuous variables involved in an interaction are centred. This is done by subtracting the variable's sample mean. | It is suggested (Aiken and West, 1991) that any continuous variables involved in an interaction are centred. This is done by subtracting the variable's sample mean. |
How do I test for an interaction involving a continuous variable?
Occasionally you might want to test an interaction involving at least one continuous variable in a regression. This involves obtaining the product of the continuous variable with any other terms in the interaction. This term may then be fitted in a regression also including lower order combinations of the variables involved in the interactions.
It is suggested (Aiken and West, 1991) that any continuous variables involved in an interaction are centred. This is done by subtracting the variable's sample mean.
Perhaps, the easiest way to do this in SPSS is to use the anova procedures available using the menu procedures under analyze>general linear model. These anova procedures allow you to create these product terms.
Examples involving continuous variable interactions are available:
[http://online.mq.edu.au/pub/PSYSTAT/contint.pdf two way Interaction between one categorical and one continuous variable.]
[http://www.psychwiki.com/wiki/Interaction_between_two_continuous_variables Interaction between two continuous variables.] The Aiken and West (1991) reference in this wikipaedia article is available from the CBSU library.