I'm trying to reduce all stats to a few simple procedures that
students can do EASILY with available stats packages. A two-way
ANOVA or an ANCOVA is as complex as I want to go. I thought SPSS
would do the trick, but I was amazed to discover that it can't.
Here's the example. I want students to convert repeated-measures
data into unpaired t tests or non-repeated measures ANOVA, by using
change scores between the time points of interest. That's no problem
when there is just the group effect: the analysis becomes a simple
unpaired t test. But when you have an extra between-subjects effect
(e.g. males and females in the treatment and control groups) it
becomes a two-way ANOVA. You make a column of change scores between
the time points of interest (e.g., post and pre), and that's your
dependent variable. The two independent effects are group (exptal
and control, say) and sex (male and female). The group term gives
the effect of the treatment averaged for males and females. Again,
no problem there, but what I want is an appropriate customized
contrast of the interaction term, which yields the difference in the
overall effect between males and females. SPSS version 10 can't do
it. I checked the on-line help, and it looks like you have to use
the command language. Well really, what student is going to manage
that? It's out of the question. Sure, you can get a p value for the
interaction, but I want confidence limits for the difference between
males and females. I've got my students to convert the p value, the
degrees of freedom, and the observed value of the effect into
confidence limits, but I shouldn't have to resort to that.
I'd also like SPSS to do an ANCOVA, but again I want to do contrasts
for the interaction, and again, they ain't there. Or did I miss
something? If so, please let me know. And can you let me know of
any simple, and preferably CHEAP or FREE, packages that will do what
I want?
Will
--
Will G Hopkins, PhD FACSM
University of Otago, Dunedin NZ
Sportscience: http://sportsci.org
A New View of Statistics: http://newstats.org
Sportscience Mail List: http://sportsci.org/forum
ACSM Stats Mail List: http://sportsci.org/acsmstats
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