Following my previous question, it sounds like the OTI contains so many different dimensions! Could you also please elaborate more about the drug use domain of the OTI for me?

Hope it helps!

Oh! I see!

The drug use assessment of OTI would ask participants when their three most recent days of drug use occurred and the drug quantify used on the last two occasions. These variables are used to calculate an average amount per day (Q), with the higher the value the greater the intake. Specifically, the intervals between days of drug use are taken as an estimate of the frequency of use, and the number of use episodes on the last two occasions is taken as an estimate of quantity consumed. This allows a simple calculation to be made based on how much of the drug they are taking and how often they are taking it. It is important to note that use on the day of the interview is not recorded, because that is not an example of a full day's use.

You could find more instructions through the below link:
https://ndarc.med.unsw.edu.au/sites/default/files/ndarc/resources/TR.011.pdf