It is a data of which records adverse events of a medical study. Basically, the medical investigator recorded every reported events and give them a event number, which is the "Current" in my data above. They also use another variable "Previous" to record if a event is actually a subsequent one from the another event, as you can see a chain of events like 6->7->8. The chains, I must say, have no apparant set pattern as the investigator sometimes enter events afterward even they are the earlier one, so I have some chains of events like 33->35->30->26. Event 26 is actually the beginning of that chain of events. Now, I need to group each chain of events by its first event. So, 6->7->8 all in group 6, 21->25 in group 21, 33->35->30->26 in group 33.
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