Paul R. Swank <[EMAIL PROTECTED]> wrote:
: Some years ago I did a simulation on the pretest-posttest control group
: design lokking at three methods of analysis, ANCOVA, repeated measures
: ANOVA, and treatment by block factorial ANOVA (blocking on the pretest using
: a median split).
I found that that with typical sample sizes, the repeated
: measures ANOVA was a bit more powerful than the ANCOVA procedure when the
: correlation between pretest and posttest was fairly high (say .90). As noted
I tried to
: publish the results at the time but aimed a bit too high and received such a
: scathing review (what kind of idiot would do this kind of study?) that I
: shoved it a drawer and it has never seen the light of day since.
You did good.
Median splits are always dumb while a test of the change scores will only
be more powerful than ANCOVA if the regression coefficient is near
1. Usually the reg coeff is about the same as the corrrelation since the
sds are likely to be about the same.
Hence ALWAYS use ANCOVA with random assignment to groups
=================================================================
Instructions for joining and leaving this list and remarks about
the problem of INAPPROPRIATE MESSAGES are available at
http://jse.stat.ncsu.edu/
=================================================================
