re: st: repeated anova

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

.

> I am struggling with this seemly simple ANOVA, but I
> can't get it set correctly. I have 40 students
> randomly allocated to 4 experimental stress groups.
> The diastolic blood pressure was measured for each
> student at baseline and at five additional times. The
> lay out is something like this:
>
> . table  time group
>
> ----------------------------------
>           |         group
>      time |    0     1     2     3
> ----------+-----------------------
>         0 |   10    10    10    10
>         1 |   10    10    10    10
>         2 |   10    10    10    10
>         3 |   10    10    10    10
>         4 |   10    10    10    10
>         5 |   10    10    10    10
> ----------------------------------
>
> Note that there are 10 students in each -group-, but
> each student was measured at 6 different times. Could
> someone please tell me what is the correct Stata
> command for this repeated ANOVA?
This is a simple split plot design that shows up everywhere in  
biology. You've got one between-subjects factor (group), and one  
within-subjects factor (time). You can analyze this design using a  
number of Stata commands: anova, manova, or xtmixed. Take a look at  
the UCLA page which covers classical designs using ANOVA and the  
equivalent xtmixed command:

http://www.ats.ucla.edu/stat/stata/faq/xtmixed.htm

On that page you will see this relevant part:

/* spf-2.4    subjects nested in a */
/* between subjects: a             */
/* within  subjects: b             */

use http://www.ats.ucla.edu/stat/stata/examples/kirk/spf2-4, clear

/* anova code: */ anova y a / s|a b a*b /, repeated(b)

xi3 e.a*e.b

xtmixed y _Ia_2 _Ib_2 _Ib_3 _Ib_4 _Ia2Xb2 _Ia2Xb3 _Ia2Xb4 || s:

test _Ia_2

test _Ib_2 _Ib_3 _Ib_4

/* scale chi^2 as F-ratio to compare with anova */
display r(chi2)/r(df)

test _Ia2Xb2 _Ia2Xb3 _Ia2Xb4

display r(chi2)/r(df)



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