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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 18:13:47 +0100

I was just working from what you wrote, for example: 

"I should have mentioned that patients who had three visits, for 
instance, have three observations, and those with two visits have two 

So counting observations and counting visits looked like the same
problem to me. 


Shehzad Ali

Thanks, Nick. But I am not trying to count the total number of
per patient but the total number of visits (varname: clinic) across all 
time points for each patient (I tried to clearly state it in the first
- sorry if I wasn't clear).

The solution I am now using is:

bysort patient_id: egen sum_clinic = sum(clinic)

On Apr 15 2009, Nick Cox wrote:

>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
>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
>>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
>> 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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