[R] User error in calling predict/model.frame

[Joshua Wiley]

On Sat, Jan 29, 2011 at 1:23 PM, Joshua Wiley <jwiley.psych at gmail.com> wrote:
>> xxAB <- dat$xxA * dat$xxB
>> meanAB <- meanA * meanB
>> sdAB <- sqrt(sdA^2 + sdB^2 + meanA^2 * sdB^2 + meanB^2 * sdA^2)

don't know why I did that the hard way (besides I think I messed up
the formula slightly), but the point is there's still a difference
between the scaled interaction from unscaled variables and the
interaction of scaled variables.

sdAB <- sd(xxAB) # <-- what I should have done

> (xxAB - meanAB)/sdAB
 [1]  0.18412267 -0.97105894 -1.45088077 -1.37131900  0.09339332  0.74663645
 [7] -1.27092403 -0.41183471 -1.45890639  0.06568750  0.60130106  1.35945072
[13]  0.79255938  1.80314145  0.51553234 -0.23553700 -0.42045914 -0.73543976
[19]  1.40009474  0.68251640
>> (sxxAB <- with(dat, scale(xxA) * scale(xxB))[1:20,])
>  [1]  0.01838999  0.25899229 -0.15337178  1.07536361 -0.26520706  0.25119036
>  [7] -0.11295999  0.05205466 -1.49676116 -0.14367368 -1.86921225  0.80860167
> [13] -0.44816896  1.44122535 -0.73361472 -0.14930187 -1.46192081  0.19784309
> [19]  0.62089372  0.0821291515