I'm rusty on my stats. I am wondering what the prerequisites are for including covariates in my regression. I'm not looking for a prediction model but just the effect of the "treat". The sample was randomized and the demographics appear pretty similar with a couple of exceptions.

Do I include all or only those that are the same in both treatment groups? Do I need to first do a statistical test to compare the two groups?

Few of the demographics appear to make a significant difference in the regression, but if I exclude them this hurts the significance level of the treat coefficient.

  • $\begingroup$ Is it a "randomized clinical trial"? Can you give more information on your dataset? $\endgroup$ – chl Oct 12 '12 at 22:31
  • $\begingroup$ It is a randomized comparison trial. Two different groups received two educational treatments: a game and a information pamphlet. We have n=207. Both groups took the same knowledge test before and after the treatment. $\endgroup$ – Jeff Oct 15 '12 at 14:20

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