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st: time in xtmixed

From   megan rossi <>
To   <>
Subject   st: time in xtmixed
Date   Mon, 25 Mar 2013 23:12:21 +1000

How is this for confusing, when time is as a continuous variable (ie. option a)the association between depvar and indepvar is significant (time is borderline sig p=0.055), however as a categorical variable, option b, the relationship between depvar and indepvar becomes insignificant...and time 1 and 2 become very insignificant (p=0.55 and 0.17)
In both scenario's the overall significance of the model is <0.0001 and log likelihoods are the same. The interaction between indepvar and time is only significant at time 0 not 1 or 2.
If I did go with option a, could I really say that independent of time depvar and indepvar are association?
Megan Rossi APD 
PhD Candidate School of Medicine, University of Queensland
Accredited Practicing Dietitian
Princess Alexandra Hospital Nutrition & Dietetic Department
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> Date: Mon, 25 Mar 2013 07:22:51 -0400
> Subject: Re: st: time in xtmixed
> From:
> To:
> Megan,
> To expand on Maarten's reply, i.time will give you separate effects
> for the second and third time points (and the effect at the first time
> point will be part of the constant term). If you use time as a
> "continuous" predictor, its coefficient will be the slope of depvar
> against time (adjusted for the contribution of indepvar), and the
> constant term will correspond to time = 0. The second model is
> simpler, but it assumes that the contribution of time is linear.
> David Hoaglin
> On Mon, Mar 25, 2013 at 3:06 AM, megan rossi <> wrote:
> > Hi Stata Guru's,
> > Can anyone advise me on whether I need to include time variable as a categorical variable (ie. have a prefix i.) in xtmixed or can it be treated as a continous variable even though the data was collected at only three time points (evenly spaced). My research question: is depvar associated with indepvar independent of time ie. is syntax a or b most appropriate:
> > a) xtmixed depvar indepvar time||ptid:time,cov(uns)
> > or
> > b) xtmixed depvar indepvar i.time||ptid:time,cov(uns)
> > I noticed that the stata youtube video on this doesn't include i. however it was from 9 time points, not 3.
> > Cheers,
> > Megan Rossi APD
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