Re: st: How to apply and interpret lincom function as an additional analysis to xtmixed

[Maarten Buis <maartenlbuis@gmail.com>]

I would use the factor variable notation instead of the outdated -xi:-
prefix. This is simple, you replace

xi: xtmixed measurement i.time*i.group || id: ,mle

with

xtmixed measurement i.time##i.group || id: ,mle

After that you can use -contrast-, -pwcompare- and -margins- to get at
the kind of thing you are looking for. For more details see: -help
fvvarlist-, -help margins-, -help contrast-, -help pwcompare-, and
-help margins_pwcompare-.

Hope this helps,
Maarten

On Fri, Jan 3, 2014 at 5:02 PM, Jakob Nielsen <jakniel@hotmail.com> wrote:
> Im using tmixed and need to make a post estimation analysis (lincom) in
>  addition to the information I get from the xtmixed analysis. However, I
>  have doubt how to write the command correctly.
>
> I am using Stata 12.1 for windows.
>
> -----
>
> Firstly,
>  my dataset consists of two groups performing two different
> interventions. There is an uneven number of measurement points between
> the groups (4 vs. 3), but the first three measurements have been
> performed at the same time between the groups.
>
> Group 1        Group 2
> id    Trial 1 Trial 2 Trial 3 Trial 4       Id    Trial 1 Trial 2 Trial 3
> FP1    x.xx    x.xx    x.xx    x.xx        FP13    x.xx    x.xx    x.xx
> Fp2    x.xx    x.xx    x.xx    x.xx        Fp14    x.xx    x.xx    x.xx
> FP3    x.xx    x.xx    x.xx    x.xx        FP15    x.xx    x.xx    x.xx
> FP4    x.xx    x.xx    x.xx    x.xx        FP16    x.xx    x.xx    x.xx
> FP5    x.xx    x.xx    x.xx    x.xx        FP19    x.xx    x.xx    x.xx
> FP6    x.xx    x.xx    x.xx    x.xx        FP20    x.xx    x.xx    x.xx
> FP7    x.xx    x.xx    x.xx    x.xx        FP21    x.xx    x.xx    x.xx
> FP9    x.xx    x.xx    x.xx    x.xx
> FP10   x.xx    x.xx    x.xx    x.xx
> FP11   x.xx    x.xx    x.xx    x.xx
>
> When I compute the xtmixed analysis, I have the above data on the long format, and I get an output looking like this:
>
> ----
> . xi: xtmixed measurement i.time*i.group || id: ,mle
> i.time            _Itime_1-4          (naturally coded; _Itime_1 omitted)
> i.group           _Igroup_1-2         (naturally coded; _Igroup_1 omitted)
> i.time*i.group    _ItimXgro_#_#       (coded as above)
> note: _ItimXgro_4_2 omitted because of collinearity
>
> Performing EM optimization:
>
> Performing gradient-based optimization:
>
> Iteration 0:   log likelihood =  68.201963
> Iteration 1:   log likelihood =  68.202082
> Iteration 2:   log likelihood =  68.202082
>
> Computing standard errors:
>
> Mixed-effects ML regression                     Number of obs      =        60
> Group variable: id                              Number of groups   =        17
>
>                                                 Obs per group: min =         3
>                                                 avg                =       3.5
>                                                 max                =         4
>
>
>                                                 Wald chi2(6)       =    164.59
> Log likelihood =  68.202082                     Prob> chi2        =    0.0000
>
> -------------------------------------------------------------------------------
>   measurement |      Coef.   Std. Err.    z       P>|z|     [95% Conf. Interval]
> --------------+----------------------------------------------------------------
>      _Itime_2 |  .3080587    .032767     9.40     0.000     .2438365    .3722809
>      _Itime_3 |  .257        .0317744    8.09     0.000     .1947233    .3192767
>      _Itime_4 |  .238        .0317744    7.49     0.000     .1757233    .3002767
>     _Igroup_2 |  .0041429    .0391134    0.11     0.916    -.0725181    .0808038
> _ItimXgro_2_2 | -.3109158    .0501596   -6.20     0.000    -.4092268   -.2126049
> _ItimXgro_3_2 | -.2541429    .0495169   -5.13     0.000    -.3511941   -.1570916
> _ItimXgro_4_2 |    0  (omitted)
>         _cons |  .103        .0250987    4.10     0.000     .0538075    .1521925
> -------------------------------------------------------------------------------
>
> ------------------------------------------------------------------------------
>   Random-effects Parameters  |   Estimate   Std. Err.     [95% Conf. Interval]
> -----------------------------+------------------------------------------------
> id: Identity                 |
>                    sd(_cons) |   .0353746   .0129137      .0172964    .0723481
> -----------------------------+------------------------------------------------
>                 sd(Residual) |   .0710498   .0075191      .0577405    .0874268
> ------------------------------------------------------------------------------
> LR test vs. linear regression: chibar2(01) =     3.15 Prob>= chibar2 = 0.0379
>
> .----
>
> This
>  is all fine. However in addition to this, I would like to get a
> comparison within group comparison of the timepoints, which is not
> provided with the xtmixed analysis.
>
> That being:
>
> Group 1: time 2 and 3
> Group 1: time 2 and 4
> Group 1: time 3 and 4
>
> And
>
> Group 2: time 1 and 2
> Group 2: time 1 and 3
> Group 2: time 2 and 3
>
> I have been told that the command to use is the following lincom commands:
>
> Group 1:
>
> lincom _Itime_2 + _Itime_3
>
>  ( 1)  [measurement]_Itime_2 + [measurement]_Itime_3 = 0
>
> ------------------------------------------------------------------------------
>  measurement |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
> -------------+----------------------------------------------------------------
>          (1) |   .5650587   .0556139    10.16   0.000     .4560574    .6740599
> ------------------------------------------------------------------------------
>
> This should compare time 2 with time 3 in group 1.
>
> Group 2:
>
> lincom _Itime_2 + _ItimXgro_3_2
>
>  ( 1)  [measurement]_Itime_2 + [measurement]_ItimXgro_3_2 = 0
>
> ------------------------------------------------------------------------------
>  measurement |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
> -------------+----------------------------------------------------------------
>          (1) |   .0539158   .0501596     1.07   0.282    -.0443951    .1522268
> ------------------------------------------------------------------------------
>
> This should compare time 2 with time 3 in group 2.
>
> However,
>  I am not entirely sure, whether I have setup the command correctly.
> Thus, the question is, whether the above lincom command is correct setup
>  in regards to the purpose (within group comparisons of time 2 and 3)?
>
> Thanks in advance.
>
> Best, Jakob Nielsen
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/faqs/resources/statalist-faq/
> *   http://www.ats.ucla.edu/stat/stata/



-- 
---------------------------------
Maarten L. Buis
WZB
Reichpietschufer 50
10785 Berlin
Germany

http://www.maartenbuis.nl
---------------------------------
*
*   For searches and help try:
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