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Re: st: Analysis of experiment involving baseline measurements


From   Ángel Rodríguez Laso <angelrlaso@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Analysis of experiment involving baseline measurements
Date   Wed, 29 Jun 2011 21:42:03 +0200

Would not be another option to check if there is a linear or quadratic
trend which is different for each intervention group to carry out a
repeated measures ANOVA where longitudinal measures are the
within-subjects factor, group measures are the between-subjects factor
and the main interest of the analysis is the polinomial trend of the
interaction within*between factors?

Best regards,

Angel Rodriguez-Laso

2011/6/29 Clyde Schechter <clyde.schechter@einstein.yu.edu>:
> Phil Jones asks for advice in adjusting for baseline measurements when
> analyzing data with two follow-up points.
>
> You need to first think about what theory underlies the intervention and
> the implications for how the outcome score will evolve over time--the
> modeling will depend on that.  Do you expect both groups to improve from
> pre to post and continue to improve at 6wks?  If so, will they continue to
> improve at the same rate as from pre- to post-, or will there be a
> tapering off (or an acceleration)? Or do you expect the scores to
> deteriorate somewhat at 6 wks?
>
> If you -reshape- your data into long format, you can model any of these
> possibilities using -xtmixed- or -xtreg-.  The independent variables
> specification may involve a single degree-of-freedom specification of
> time, or time as a factor variable, or perhaps as a spline. And your
> representation of time will then have interaction terms with group.  You
> will also have the option of either including the baseline value as a
> covariate (and not analyzing time = pre observations) or not.  But you
> have to have a model of the time-trajectories of the output in mind to
> make the corresponding decisions.
>
> Hope this helps you make progress.
>
>
> Clyde Schechter
> Department of Family & Social Medicine
> Albert Einstein College of Medicine
> Bronx, NY, USA
>
>
> Please note new e-mail address: clyde.schechter@einstein.yu.edu
>
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