Dr. Winkler,
Thank you for your reply. I have several follow-up questions which I think will help me better understand your response.
There were baseline differences in age and dementia status between the experimental and control groups that I think might be biasing the results of the intervention. So, in my analysis, I entered baseline values for age and dementia status as "control variables" and I want to know if there are effects of the intervention above and beyond any effects of age and dementia status on FA values, for example, as these effects are fairly well established. Again, since it was a complicated 2x2 ANOVA design, my outcome variables are change scores for the DTI parameters (e.g., FA) which represent any change from pre- to post-intervention, and were used in a two-sample t-test.
Based on this information, does the contrast I listed in my first post: 1 -1 0 0 (with 1 -1 being the contrast of exp vs. control group and age and dementia status being assigned values of 0) accurately model the variables to give the desired information? In other words, would this contrast reflect larger changes in the experiential group (1) than the control group (-1), above and beyond any effect of age and dementia status on the change scores?
Again, I was expecting that controlling for baseline age and dementia status would reduce the significant of the results, given that there were significant baseline group differences in these two variables. However, the results did not change very much, leading me to wonder whether the EVs and/or contrasts were set up incorrectly.
I am also not sure I understand your statement that TFCE statistics would always be positive. FA is positive, but if it decreased across the year in the study, which I might expect with older adult populations, the change scores would be negative, and I assumed the t-statistic would be negative as well, reflecting the deterioration of FA. (I calculated all the change scores as time 2 minus time 1).
Thank you for your time and help!
Best,
Catherine Mewborn