- To track drug use, stress, and geographical location (a measure of environmental risk), each participant will carry a PDA and a GPS unit for 16 of the 18 weeks. Participants will make entries (1) each time that they use a drug and (2) each time they feel overwhelmed, anxious, or stressed more than usual. Participants will also make three random-signal-triggered recordings per day and one brief (end of day) recording.