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RE: st: AW: simple sum() question

From   "Nick Cox" <>
To   <>
Subject   RE: st: AW: simple sum() question
Date   Wed, 15 Apr 2009 17:07:28 +0100

Unless there are further complications as yet unrevealed, 

bysort id : gen visits = _N 

is a direct and simple solution. 

If you just wanted to count a subset, then 

gen interesting = <binary variable defining interesting> 
bysort interesting id : gen interesting_visits = _N if interesting 

There are -egen- routes as well, but for problems like this going back
to basics is difficult to beat. 

See also 

SJ-2-1  pr0004  . . . . . . . . . . Speaking Stata:  How to move step
by: step
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N.
J. Cox
        Q1/02   SJ 2(1):86--102                                  (no
        explains the use of the by varlist : construct to tackle
        a variety of problems with group structure, ranging from
        simple calculations for each of several groups to more
        advanced manipulations that use the built-in _n and _N

if a tutorial is needed. That's free on-line at the Stata Journal

Note that even if you did want a collapsed dataset, -contract- rather
than -collapse- is more direct. 


Shehzad Ali

Hi Martin and Josiane,

Thank you for your replies. You are right that I am interested in the
count of visits for each patient and not the running sum.

Sorry, I should have mentioned that patients who had three visits, for 
instance, have three observations, and those with two visits have two 
observations. Therefore, the total number of observations for 100
is less than 400 (I had made up hypothetical numbers in haste to
the case. Not always a good idea).

With Martin's solution, I will need to have four observations for each 
patient (sorry this was my fault as I didn't provide the correct 
information). With Josiane's suggestion, the dataset collapses which is
what I want.

Can you suggest a modified solution please? Again, sorry for the unclear

email earlier.

On Apr 15 2009, Martin Weiss wrote:

> I am betting that you want a count of visits, not a running sum, but 
> correct me if I am wrong...

>set obs 400
>egen float patient = seq(), from(1) to(400) block(4)
>egen float visit = seq(), from(1) to(4) block(1)
>//not strictly necessary
>xtset patient visit
>//less than 4 visits for some
>replace visit =. if runiform()<0.05
>bys patient: egen overallvisits=count(visit)
>l in 1/20, sepby(patient) noo

Shehzad Ali

>I have a simple question about summing across observations. I have 100 
>patients (variable: patient_id) in the dataset, each had clinic visits 
>(variable: clinic) and hospital visits (variable: hospital) recorded at

>weeks 4, 8, 12 and 16. The dataset is long and hence I have 400 
>observations (one observation per patient per time point).
> I want to sum the clinic visits for each patient (across all 4 visits)

> bearing in mind that some patients had less than 4 visits. So
> I want to generate a new variable that will produce the sum of clinic 
> visits for each patient.

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