[alfus21]

They take a large sample population, and get the answers of teenagers from all over the country.

With such a large number of in the sample, the margin of error calculated (Population Size/Sample Size) is very small resulting in a very close estimate.

Much like the Presidential Elections in which several thousand are asked who they would vote for, the margin of error usually 3% is always there.

The group that does most of these studies on teenagers and drug abuse is MTF(Monitoring the Future) They take samples from 8th, 10th, and 12th graders and ask of their use or contact within the last day, 30 days, 6 months, and lifetimes I think.

Here's their website.
http://www.monitoringthefuture.org/