At 06:50 PM 4/2/01 +0100, Dr Graham D Smith wrote:
>Thinking about these issues has caused me to reassess the assumptions
>underpinning the use of the repeated measures t test (for differences).
>For a long time, I have thought that the homogeneity of variance
>assumption is meaningless for the RM t test. In other words there is no
>point in comparing the variability of scores from one condition with the
>variability of scores in the other condition prior to using the test. I
>thought this because, once the difference scores are calculated
>homogeneity of variance is meaningless. The t test is performed on the
>differences not the scores themselves whose variances may differ (so
>what?). However, I now wonder whether in fact one should look at
>homoscedasticity of the relationship between the difference of the scores
>in the two conditions and the sum of the scores in the two conditions; for
>example, for my data the relationship between Incong-Cong and Incong+Cong.
>(Actually the data from my study were not clearly heteroscedastic).
let's say that you do a pre and post study with the same Ss ... say,
pretest score and posttest score ... AND, while there is variance at pre
... all Ss master the material and, the variance on scores on the post more
or less goes away (a not uncommon problem in mastery learning studies)
are you suggesting that the difference in variances at pre and post should
be of no concern when doing a dependent t test on the means?
=================================================================
Instructions for joining and leaving this list and remarks about
the problem of INAPPROPRIATE MESSAGES are available at
http://jse.stat.ncsu.edu/
=================================================================
