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st: Multilevel nested data structure

From   Mohammed Mohammed <>
To   "" <>
Subject   st: Multilevel nested data structure
Date   Thu, 8 Nov 2012 22:51:31 +0000

Dear stata listers

My data structure has several levels of nesting from a study where patients measure their own blood pressure measurements.

The nesting structure is as follows:-

Family Practice/Patient/Day/blood pressure readings.

Patients are nested within a family practice and over an 8 day period patients measure their blood pressure (multiple readings per day).

I am using xtmixed to identify predictors of the blood pressure, thus:

xtmixed systolic age male || practice_code: || patient_id: || day:

The output from xtmixed suggests  there are 28 family practices (which is correct) and 752 patients (which is correct) and 2225 days (which is not correct as each patient was in the study for 8 days).

I am not sure why the group table at the header of the xtmixed output does not give the proper number of groups for days (as 8). 

I guess i must be doing something wrong....any advice would be much appreciated...

best wishes

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