ABSTRACT
Longitudinal data can be coded into “long” and “wide” formats. A wide dataset will have one record for each individual. The observations made at different time points are coded as different columns. In the wide format every measure that varies in time occupies a set of columns. In the long format there will be multiple records for each individual. Some variables that do not vary in time are identical in each record, whereas other variables vary across the records. The long format also needs a “time” variable that records the time in each record, and an “id” variable that groups the records from the same person.
