re: st: Specifying a repeated measures analysis in anova and xtmixed

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

.

An answer to the discrepancy below is that one model assumes no  
interaction between subject and experimental factor while the other  
is more general and does.

Cheers,

-Dave



> > 2) If I wanted to analyze this as a mixed model, what is an  
> > appropriate way to use xtmixed? I consider case and reader to be  
> > random effects, as both are drawn from larger populations of  
> > possible cases and readers. Likewise, I consider modality to be a  
> > fixed factor, as there are only N modalities of interest in any  
> > particular phase of my study. As such, I came up with the  
> > following as a rough guess of how to express it:
>
> There seems a discrepancy between the randomized block factorial  
> design anova solution given at UCLA and that given in the Stata  
> FAQ. Unless the designs are different. I can't tell from the data  
> table.
>
> http://www.stata.com/support/faqs/stat/anova2.html#half713
>
>
>      
> --------------------------------------------------------------------
>               |             10 minute time periods and dial
>               | ------- 1 ------    ------- 2 ------    ------- 3  
> ------
>       subject |    1     2     3       1     2     3       1      
> 2     3
>     ---------- 
> +---------------------------------------------------------
>             1 |   45    53    60      40    52    57      28     
> 37    46
>             2 |   35    41    50      30    37    47      25     
> 32    41
>             3 |   60    65    75      58    54    70      40     
> 47    50
>      
> --------------------------------------------------------------------
>
>
> . anova score subject period / subject*period dial / subject*dial  
> period*dial, repeated(period dial)
>
> or
>
> . anova y s a / s*a b / s*b a*b, repeated(a b)
>
> versus
>
> http://www.ats.ucla.edu/stat/stata/faq/xtmixed.htm
>
> ----------+---------------------------------------------------------
>           |                         a and b
>           | ------- 1 ------    ------- 2 ------    ------- 3 ------
>         s |    1     2     3       1     2     3       1     2     3
> ----------+---------------------------------------------------------
>         1 |   37    43    48      39    35    46      31    41    64
>         2 |   42    44    47      30    40    36      21    50    52
>         3 |   33    36    29      34    31    45      20    39    53
>         4 |   29    27    38      26    22    27      18    36    42
>         5 |   24    25    28      21    27    26      10    34    49
> ----------+---------------------------------------------------------
>
>
> / 
> ********************************************************************** 
> ********************/
> / 
> *                                                                      
>                    */
> /*              randomized block factorial  
> design                                         */
> / 
> *                                                                      
>                    */
> /* from kirk -- see http://www.ats.ucla.edu/stat/stata/examples/ 
> kirk/kirkstata10.htm      */
> / 
> *                                                                      
>                    */
> / 
> ********************************************************************** 
> ********************/
>
> use http://www.ats.ucla.edu/stat/stata/examples/kirk/rbf33, clear
>
> /* anova code: */ anova y s a b a*b, repeated(a b)
>
> xi3 e.a*e.b
>
> xtmixed y _Ia_2 _Ia_3 _Ib_2 _Ib_3 _Ia2Xb2 _Ia2Xb3 _Ia3Xb2 _Ia3Xb3  
> || s:
>
> test _Ia_2 _Ia_3
>
> /* scale chi^2 as F-ratio to compare with anova */
> display r(chi2)/r(df)
>
> test _Ib_2 _Ib_3
>
> display r(chi2)/r(df)
>
> test _Ia2Xb2 _Ia2Xb3 _Ia3Xb2 _Ia3Xb3
>
> display r(chi2)/r(df)
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