Does it count underage drinking? I didn't even notice that if it does. Regardless, around 85% of Americans use marijuana alone, so it wouldn't change the statistics by more than one or two percent. So I don't see how that would be misleading.
On page 103, it says that 82% of fifty-year-olds (who attended high school during the heaviest-drug-using period in America's recent history) have used marijuana at least once in their lives. But only 10% of that same group have used it in the last year.
Among current 18-year-olds, only 42% have ever tried marijuana even once. And among most age groups under the age of 30, only between 23-34% have used marijuana within the last year.
However, I don't see evidence that the study you posted counts underage drinking in among illicit drug use.
I said 'have used' in my original comment, which is what I meant. (In my reply I was thinking lifetime prevalence, i.e. around 85% 'use' marijuana at some point during their lives. Poorly phrased, my bad.)
"Among current 18-year-olds, only 42% have ever tried marijuana even once."
The age of first use of marijuana is highly variable. By age 25 around 65% of people have used marijuana, and more people keep trying throughout the next couple decades of their life. The important statistic is what percentage of people use marijuana at least once in their lives, not what percentage use it on any given day.
"On page 103, it says that 82% of fifty-year-olds (who attended high school during the heaviest-drug-using period in America's recent history) have used marijuana at least once in their lives."
The 82% figure doesn't count people who didn't make it to fall of their 12th grade year in high school, so you actually have to adjust it up a couple percentage points. Also, even if less high schoolers were using illegal drugs for a decade or two, that probably won't have much effect on the overall lifetime prevalence. It might eventually decline a few points, but probably not that much.
1977 has got to have been the historical peak of high school usage. I notice they don't mention 51 year olds or 49 year olds, they have specifically pulled out this one exact age for this statistic.
It then goes on to say that 29-30 yr olds have a 68% "adjusted" lifetime prevalence. What does that mean adjusted? It means the actual number they measured is lower (32% for 19-30 year olds), and they tweaked it upwards because they wanted a bigger number.
About this time is where I say a study has too many methodological flaws and should be disregarded.
Only 50-year-olds are mentioned because this is a longitudinal cohort they've been testing since 1977, and this year (2009) it's their turn to be reported.
The "adjusted" number is higher than the measured number because some people answered the survey ten years ago saying they'd tried marijuana in the past year but then in the current survey the same respondent said "no" when asked if they'd ever tried marijuana in their lifetime. So they're publishing both the actual numbers and the "we know you're probably lying" numbers.
It's explained the study.
"About this time is where I say a study has too many methodological flaws and should be disregarded."
Try actually reading it. It's really not that difficult to understand. The 'maturity recanting effect' section here also gives a brief summary of what they did and why:
http://www.erowid.org/psychoactives/statistics/statistics_ar...
Don't criticize studies without taking the time to understand them, otherwise you just sound like these people:
It's also charming that you cite erowid.org as a reliable unbiased source. I am familiar with the site and have read it. It is a radical pro-drug site.
Even their legal articles don't take any position on what drug policy should be, all they do is explain the case law that exists.