1. The sample - Notes

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1. A selected nominee was later asked in one of the first questions if he or she had used cocaine more than 25 times. If the answer was `no' the interview had to be stopped. This was never necessary.
2. In a household survey in Amsterdam of the population of 12 years and older, current use of cocaine in the age cohort under 20 years of age appeared to be less than 0.5% (). This explains the difficulty of finding experienced cocaine users in this age cohort by our field workers.
3. This assumption is quite arbitrary. It saves us from the problem of including experimental users of cocaine of a long time ago in the reference group from the household survey. It does not save us from including experimental users from a short time ago! This comparison should be considered as a rough tool, but there is at least some possibility to check our snowball sample against another (independent) sample.
4. In the Dutch school system, children go to elementary school from 4-12 years of age (LO). After that, there is a differentiated and rather complicated system of various branches and levels. The lowest level of the technical branch is called is the 4-year LBO (Lower Profession Education, e.g. plumber) and is the lowest level of the technical branch, followed by a 3-year MBO (Middle Profession Education, e.g. nurse) and a 4-year HBO (Higher Profession Education; e.g. social worker, engineer). The basic level of the more general branch starts with the 4-year Mavo at the lowest level, the 5-year Havo at medium level and the 6-year Atheneum or Gymnasium at the highest level. Only the last two schools prepare for University. Many courses run at American universities, are not taught at Dutch universities, but at MBO and HBO level.
5. The word 'risky' was offered to our respondents, without any specification. If respondents would count nominees as risky users or not was purely a subjective matter.