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From | David Hoaglin <dchoaglin@gmail.com> |
To | statalist@hsphsun2.harvard.edu |
Subject | Re: st: time in xtmixed |
Date | Mon, 25 Mar 2013 07:22:51 -0400 |
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 <megan_rossi@msn.com> 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 * * 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/