"There are still no significant differences in marijuana use or the use of other illicit drugs as a function of whether or not the school has (a) drug testing of any kind, (b) drug testing of students based on cause or suspicion, or (c) drug testing of athletes. Nor is there evidence that the heavy drug-using segment of the student population, specifically, is deterred from using marijuana or other illicit drugs by random or for-cause testing."
Source
Yamaguchi, Ryoko; Johnston, Lloyd D.; and O’Malley, Patrick M., "Drug Testing in Schools: Policies, Practices, and Association With Student Drug Use," Institute for Social Research (Ann Arbor, MI: The University of Michigan, 2003), p. 15.
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