I am running a multiple linear regression with a continuous DV and a number of independent variables, one of which is ordinal (three-levels).
I am trying to follow David J Pasta’s instructions to evaluate the opportunity of treating the ordinal variable as continuous: [support.sas.com/resources/papers/proceedings09/248-2009.pdf]
However, the actual example comes from SAS and I am unable to do the same thing with SPSS (which I am familiar with) or R (which I am learning to use).
He suggests entering both the categorical) and linear (continuous) version of the variable in the same model in order to test the deviation from linearity (I have pasted an excerpt from the paper at the end of the question)
However, when I enter both the categorical variable (coded as two dummy variables) and the linear variable in the model, SPSS drops either the linear variable or one of the dummy variables. The same thing happens if I code the categorical variable as three dummy variables. I guess this is due to multicollinearity, but maybe I am just doing something silly and I am unaware of it..
“…Then for each variable you could include both the original (categorical) version and the linear version in the same model! Won't that be redundant? Yes, it will, and the linear version will show up with zero degrees of freedom and not statistical test of significance. The Type III statistical test of the categorical version will have one less degree of freedom than usual and it will be testing the deviation from linearity – whether the remaining K-2 df have statistically significant explanatory power. If the categorical version is statistically significant, that tells you there is a significant non-linear component and it makes sense to omit the linear version of the variable. If the categorical version is not statistically significant by whatever criterion you shoose to use, that means that the linear component carries the explanatory power and the categorical variable can be dropped. (Of course, there's no guarantee that the linear version will be significant after dropping the categorical variable – that still needs to be tested.)”
Thanks for any help!