re: Re: st: re: ANCOVA for pre post designs

[David Airey <david.airey@vanderbilt.edu>]

Joseph Coveney !!!kindly!!! showed that in fact I can use ANCOVA in a 
within-subjects design. His .do file would be a nice addition to 
Higbee's ANOVA FAQs on the Stata site, once I help with the equivalent 
MANOVAs.

It is very typical of people in my field to still use ANOVA rather than 
MANOVA for repeated measures analysis. Nor do I often seen mixed model 
approaches that allow modeling the covariance structure explicitly, 
like in SAS Mixed or R LME/NLME, despite the benefits of these 
approaches. I have read enough about the disadvantages of ANOVA that I 
have spent some time learning the basics of MANOVA and procedures like 
R LME for repeated measures. I hope it pays off.

In my behavioral experiment, I have 2 levels in each factor I've 
manipulated. So for some analyses, use of ANOVA or MANOVA is equivalent 
since sphericity is always met if all factors have 2 levels.

-Dave


> I believe that you *can* use the -continouous()- and -repeated()- 
> options
> together.  With involved models, you might need to explicitly specify 
> the
> between-subjects error term using the -bse()- option, but that's not 
> any
> impediment.  I've illustrated their combined use in the do-file below 
> using
> the two repeated-measures ANCOVA examples from Chapter 10 of B. J. 
> Winer,
> D. R. Brown & K. M. Michels, _Statistical Principles in Experimental 
> Design_
> 3rd Edition. (NY: McGraw-Hill, 1991), pp. 820-36.  The first example 
> has a
> single time-invariate covariate for each individual; the second has a
> time-varying covariate (i.e., the covariate has a different value at 
> each
> repeated measurement).  Note that the two examples in Winer, Brown & 
> Michels
> don't need the -repeated()- option, since they both have only two
> observations.
>
> I'm not sure why Stata Corp. recommended considering -manova- to 
> David, but
> perhaps it's because of the admonition stated in the preface to the
> repeated-measures ANCOVA section from Winer, Brown & Michels (Section 
> 10.5,
> p. 820), "In the behavioral sciences area, the covariance matrix for 
> the
> repeated measures is very likely to be more complicated than the
> corresponding covariance structure assumed for the usual split-plot 
> design
> [in agricultural research]."  David's original posting related to the 
> area
> of schizophrenia and sensorimotor inhibition.
>
> Joseph Coveney
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