st: simple sum() question

[Shehzad Ali <sia500@york.ac.uk>]

Hi,

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 effectively I 
want to generate a new variable that will produce the sum of clinic visits 
for each patient.

Any help would be appreciated.

Thank you,
Shehzad
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