CODEWITHSUNDEEP
databaseredcap
C Tor
C Tor

Reputation: 21

How do I reference a field from a repeatable instrument in REDCap?

I'm trying to use datediff() to calculate age in a longitudinal REDCap database, but the function is returning [no value], despite the calculation being valid and the smart variable help page corroborating that the function seems correct.

The first date is in a non-repeating instrument in one event. The second date, and also where the calculation is being done, is in a field in a second, repeatable instrument, in a separate, non-repeatable event.

My calculation currently looks like this:

datediff([firstdate],[seconddate][current-instance], "y")

I've also (for lack of any idea how to fix it), tried 

datediff([firstdate],[secondeventname][seconddate], "y")

Both calculations return [no value]. I've double checked that the dates are in the same ymd format, and that the function DOES work when I replace the second argument with 'today', so I know that the issue is the second argument, but the smart variable FAQ seems to be suggesting the first line of code above, which of course hasn't been working.

Does anyone have experience with what the issue might be?

Upvotes: 1

Views: 2055

Answers (1)

Jangari
Jangari

Reputation: 868

In a longitudinal data collection project, you should prefix your variables with the event that it comes from, otherwise REDCap will only look into the current event for that variable, and return no value if it can't find anything.

Furthermore, the datediff function takes a 4th parameter for the date format, either "ymd", "dmy" or "mdy", and both date1 and date2 must be in the same format.

You may not need the smart variable for current-instance, at least in my testing for this I didn't need it, since if you are performing this calculation from the event that contains [seconddate], indeed from the instance if it is repeating, then you might only need to use [seconddate] to reference it, whereas to reference [firstdate] you need to prefix it with [event_1_arm_1] or whatever your event name is, or the smart variable [first-event-name] (which would be much more portable for multi-arm studies).

So I would try the following:

datediff( [first-event-name][firstdate], [seconddate], "y", "ymd" )

Upvotes: 1

Related Questions