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From | rgutierrez@stata.com (Roberto G. Gutierrez, StataCorp) |
To | statalist@hsphsun2.harvard.edu |
Subject | Re: st: unique IDs in xtmixed |
Date | Fri, 20 May 2011 13:22:13 -0500 |
Teya Rutherford <teyarutherford@gmail.com> asks: > I have encountered a strange thing in my analysis. I am working with > long data that has between one and three years of information per > student. I created an analysis sample to limit my analysis to those > students without missing data on certain variables and who are present > during our years of interest (years 2 & 3) for at least one year. When I > codebook ID within this sample I get about 28K unique IDs. I've > confirmed this number by using sum with this sample by year. > When I run xtmixed (code below) it says that there are 30236 unique > students (based on the same ID variable) contributing between 1 & 3 > years to the data. The if statement appears to work, because it keeps > the numbers consistent between my conditional and unconditional models > (before adding the statement it was not consistent). I've never before > *gained* observations from codebook to analysis and I'm extremely > puzzled. Can anyone help? > Code for unconditional model: > xtmixed SSMATH c_grade || MINDSH: c_grade || ssid: c_grade if > atleastoneyear == 1 & analysissample == 1, mle var cov(un) > ----------------------------------------------------------- > | No. of Observations per Group > Group Variable | Groups Minimum Average Maximum > ----------------+------------------------------------------ > MINDSH | 52 433 1093.7 1985 > ssid | 30236 1 1.9 3 > ----------------------------------------------------------- What Teya is observing is that -xtmixed- treats model levels as nested despite how the group IDs are coded. While -ssid- may have 28K unique values when considered in isolation, in an -xtmixed- model where -ssid- is nested within -MINDSH-, it can have more unique values. That is because an -ssid- coding of 12 (say) within -MINDSH==1- is treated as distinct from and -ssid- coding of 12 within -MINDSH==2-, or any other value of -MINDSH-. It's just the nature of the nesting. --Bobby rgutierrez@stata.com * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/