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Re: st: simple sum() question


From   Ulrich Kohler <kohler@wzb.eu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: simple sum() question
Date   Fri, 17 Apr 2009 15:42:56 +0200

Shehzad Ali wrote
> Here is a quick summary of what I am doing. Each patient (varname: 
> patient_id) was observed at 4 time points and at each time point we asked 
> about the clinic visits (varname: clinic) in the last 3 months. The dataset 
> is in long form (shown below):
> 
> patient_id    timepoint    clinic 
> 1                 1           0
> 1                 2           1
> 1                 3           .
> 2                 1           2
> 2                 2           0
> 3                 1           1
> 3                 2           .
> 
> The line below generates a sum of all clinic visits for each patient:
> 
> bysort patient_id: egen sum_clinic = sum(clinic)
> 
> Now if at one time point, clinic visit is missing (as its seen for patients 
> 1 and 3), then I want stata to return missing value for the sum. The above 
> command returns the total of the non-missing observations, ignoring the 
> missing ones (understandably). But if I tried:
> 
> bysort patient_id: egen sum_clinic = sum(clinic) if clinic!=.
> 
> then it returns missing value for the sum variable only for the time point 
> which is missing and not for all the time points for that patient. Can 
> anyone please suggest how to resolve this?

Martin already gave a good solution. Here is an alternative starting
from first principles:

. by patient_id (clinic), sort: gen sum_clinic = sum(clinic)

. by patient_id (clinic): ///
 replace sum_clinic = cond(!mi(clinic[_N]),sum_clinic[_N],.)

This uses the fact that observations which are missing on clinic==. are
sorted at the end of each observation. 

> Secondly, whats the best way to collapse the dataset to one observation per 
> patient? Once I have the sum_clinic for each patient, it would be easier 
> just to have one observation per patient.

by patient_id: keep if _n==1


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