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Re: st: unique IDs in xtmixed
From
[email protected] (Roberto G. Gutierrez, StataCorp)
To
[email protected]
Subject
Re: st: unique IDs in xtmixed
Date
Fri, 20 May 2011 13:22:13 -0500
Teya Rutherford <[email protected]> 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
[email protected]
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