Hi Ashish,
I am rather more concerned about whether what you outlined is legitimate
(your question 1 below). If you are looking at children, higher AGE will
be associated with higher TIV, so both variables would essentially
measure the same thing (see Miller & Chapman, Misunderstanding Analysis
of Covariance), which I really would not like to see.
Of course, if you are looking at adults, this will probably not apply.
Best wishes,
Stephan
Ashish Ranpura schrieb:
Dear all,
I'm attempting to plot a cubic relationship between two variables
controlling for the effects of a third variable. In this short example,
I'm trying to use AGE to predict CORTEX while controlling for the
effects of TIV (total intracranial volume):
(snip)
Although the last 'lines' command fails, summary(cortex.lm) does give me
reasonable coefficients for the multiple regression. Can anyone advise
me if (1) this is at all a legitimate procedure, and (2) how to draw the
cubic function that fits the multiple regression?
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