[R-sig-ME] fixed effects correlated with the intercept

Austin Frank austin.frank at gmail.com
Sat Mar 24 19:58:15 CET 2007

On Fri, Mar 23 2007, John Maindonald wrote:

> Is'nt this what might be expected.  Center the covariate about
> its mean and, depending on the detailed variance-covariance
> structure, the correlation may well reduce to zero.
> ...
> [snip useful demonstration]
> ...
> the correlation can be made arbitrarily close to -1 or 1,
> respectively.

Thanks, this explains a lot.  I also appreciate the general point
about thinking in terms of lm when trying to reason about the fixed
effects in a model.

> What do you mean when you say "I have two covariates that I consider
> to be controls in my model." Do you mean that these code for
> observations that you are treating as controls?  Or what?

I guess more precise terms might be "post-hoc controls" or "possible
confounds".  Our stimulus selection process did not take into account
certain properties of the stimuli that may have influenced the
observed behavior.  By adding these properties into the model as
post-hoc controls we can test whether the factors of interest have a
significant effect on the observed behavior even when these other
properties of the stimuli are accounted for.

Thanks again,

Austin Frank
GPG Public Key (D7398C2F): http://aufrank.net/personal.asc

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