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From |
Jakob Nielsen <[email protected]> |

To |
"[email protected]" <[email protected]> |

Subject |
RE: st: How to apply and interpret lincom function as an additional analysis to xtmixed |

Date |
Thu, 9 Jan 2014 09:05:41 +0000 |

Dear Steve and Maarten Thanks for your comprehensive replies. It is of great help! Best, Jakob > Subject: Re: st: How to apply and interpret lincom function as an additional analysis to xtmixed > From: [email protected] > Date: Wed, 8 Jan 2014 18:05:26 -0500 > To: [email protected] > > Jakob Nielsen has two groups, 4 time points in the first, and three in > the second group. He wants to use -lincom- to get some pre-specified > comparisons (if not pre-specified, then he needs a multiple comparison > correction). > > What Jakob has "been told" to do is not correct. Here are solutions that > will do as he asked. Prefix all by -lincom- > > Group 1 > > _Itime2 - _Itime3 /* compare 2 & 3 */ > _Itime2 - _Itime4 /* compare 2 & 4 */ > _Itime3 - _Itime4 /* 3 & 4 */ > > Group 2 > > _Itime_3 +_ItimXgro_3 _2 /* 3 and 1 */ > _Itime_2 +_ItimXgro_2_2 /* 2 and 1 */ > _Itime_3 +_ItimXgro_3 _2 - (_Itime_2 +_ItimXgro_2_2 ) /* 3 and 2 */ > > > As Maarten points out this will be much easier with, -margins-, here > applied to -regress- > > > *******************START****************** > sysuse auto, clear > gen group = foreign +1 > recode rep78 1/2=3 > qui reg mpg rep78##group > /* cell means and standard errors*/ > margins rep78#group > > /* repeat margins with -coeflegend- > to see coefficient names > & -post- results to use -lincom- */ > > margins rep78#group, coeflegend post > /* Compare rep78 = 3 to rep78 = 4 for groups = 1 & 2 > Ignore the "bn" in the variable name*/ > lincom _b[3.rep78#1.group] - _b[4.rep78#1.group] > lincom _b[3.rep78#2.group] -_b[4.rep78#2.group] > > /* pwcompare: all pairwise comparisons: */ > qui reg mpg rep78##group > margins rep78#group, pwcompare > ********************STOP***************** > > > Steve > [email protected] > > On Jan 3, 2014, at 11:02 AM, Jakob Nielsen 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/ > > > * > * 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/ * * 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/

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