|
Size: 2247
Comment:
|
Size: 2250
Comment:
|
| Deletions are marked like this. | Additions are marked like this. |
| Line 15: | Line 15: |
| *[http://www.psychwiki.com/wiki/Interaction_between_two_continuous_variables Interaction between two continuous variables.] The Aiken and West (1991) reference in this wikipedia article is available from the CBSU library. They suggest following Cohen and Cohen (1983, p.323) centering the moderator variable (Z) and comparing three regression lines of X on Y at Z value equal to -1, 0 and +1 which correspond to one sd below the moderator variable mean, the mean and one sd above the mean. | *[http://www.psychwiki.com/wiki/Interaction_between_two_continuous_variables Interaction between two continuous variables.] The Aiken and West (1991) reference in this wikipedia article is available from the CBSU library. They suggest following Cohen and Cohen (1983, p.323) centering the moderator variable (Z) and comparing three regression lines of X on Y at Z values equal to -1, 0 and +1 which correspond to one sd below (the moderator variable) mean, the mean and one sd above the 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.
Although interactions involving continuous variables (called covariates in SPSS) are not fitted by default in SPSS, you can create these product terms by clicking on the model button in the GLM univariate or repeated measures options.
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 wikipedia article is available from the CBSU library. They suggest following Cohen and Cohen (1983, p.323) centering the moderator variable (Z) and comparing three regression lines of X on Y at Z values equal to -1, 0 and +1 which correspond to one sd below (the moderator variable) mean, the mean and one sd above the mean.
There is also an on-line calculator which uses the method "Simple Intercepts, Simple Slopes, and Regions of Significance in MLR 2-Way Interactions" suggested by Aiken and West (1991) described in pdf format [attachment:modregion.pdf here] and [http://www.people.ku.edu/~preacher/interact/index.html here.] This uses the regression estimates, their variances and covariances which may be obtained from any standard linear regression output e.g. in SPSS.
The covariance involving intercepts, however, is not outputted directly in SPSS (see [:FAQ/constregSPSS: here]).