Yesterday, we told you about a study showing no link between medical marijuana legalization and teen pot use.

The co-author of that study, University of Colorado Denver professor Daniel Rees, sees the findings as directly contradicting attempts to link MMJ and marijuana usage among young people that are favorite talking points among federal officials.

"There have been other studies about medical marijuana and the teenage use of marijuana," Rees acknowledges, "but they've been very limited in the number of years studied and the data they used. Some have looked at just one state, some have been a cross-section. But our study is by far the most comprehensive."

Why? Because Rees, working with Benjamin Hansen, assistant professor of economics at the University of Oregon, and D. Mark Anderson, assistant professor of economics at Montana State University, pulled their information from four separate data sets. They include the National Youth Risk Behavior Survey conducted by the Center for Disease Control; similar surveys assembled by 35 states (including all thirteen that legalized medical marijuana between 1993 and 2009, the years covered by the report); the National Longitudinal Survey of Youth; and the Treatment Episode Data Set.

The latter, known by the acronym TEDS, is especially useful for researchers, Rees feels. "It shows whether anyone admitted to a federally funded substance abuse treatment center tested positive for marijuana," he notes. "So it doesn't rely on self-reporting, as the others do. This is an actual test."

The combination of the data from assorted sources fills in gaps left by the CDC's reports, and when viewed in totality, Rees says it shows a "negatively significant result." In other words, legalization of medical marijuana "is associated with a drop in marijuana use among teenagers."

The bottom line from his perspective: "There's no evidence that marijuana use goes up after legalization -- which is, of course, what the drug czar and John Walsh are claiming."

Indeed, Walsh, the U.S. Attorney for Colorado, has frequently used such assertions to justify his office's policy of sending shutdown-threat letters to dispensaries within 1,000 feet of schools. In a March response to Boulder District Attorney Stan Garnett, who asked that this policy not be deployed in his community, Walsh wrote:

This office has reviewed information from many sources, including our public schools, as well as hospitals and medical professionals, that shows an alarming and substantial spike in marijuana abuse by children and young people during that same period. When this disturbing information came to the attention of this office, we concluded that our responsibility -- as federal law enforcement officials, and also as Coloradans living in the very Colorado communities impacted by these alarming trends -- required a response.

These assertions are contradicted by CDC data for Colorado between 2009 and 2011 -- a time period that stretches beyond the one Rees and his colleagues analyzed. As shown in the following graphic, shared by Sensible Colorado's Brian Vicente, teen marijuana use in the state actually declined during the period:

Rees has double-checked the data depicted above, and while he doesn't think more sweeping conclusions can be drawn from a single state's experiences, he says the numbers "absolutely fly in the face of any claims that teen marijuana use is up in Colorado in the last two years. It's down, and that's not according to us. It's according to the Center for Disease Control."

Should adults be allowed to use marijuana recreationally -- the goal of Amendment 64, the Regulate Marijuana Like Alcohol Act, and Initiative 70, another proposal seeking to qualify for the November ballot -- would the lack of correlation between legalization and teen pot use remain? Rees thinks so.

Page down to continue reading our interview with Daniel Rees and to see the study. For one thing, Rees doesn't see the distinction between the legalization of medical marijuana and possible approval for recreational use "to be particularly useful. There's strong anecdotal evidence that medical marijuana is finding its way into the recreational market. How could it not? So the question is: Are teenagers gaining access to that recreational market, or are they somehow banned?"

He suspects that answer will defy expectations, just as the most recent study did.

"When we went into this project, we thought teen use would go up," he concedes, "and we were stunned when the results came back the way they did. There was every reason to fear that medical marijuana would get diverted into the recreational market, the price would come down, and teens would gain access -- and I don't want to belittle that fear. But we just don't see any evidence of that."

Rees fully expects that critics will attempt to shoot holes in the study, just as they did a November 2011 paper by him and Anderson that showed a 9 percent decline in traffic fatalities in states that legalized medical marijuana, as well as a sizable dip in alcohol consumption. He acknowledges frustration over false charges that he and Anderson failed to take into account certain factors and believes some people with axes to grind may have deliberately misread the study. This time around, he hopes that "people actually read the study before they attack it."

What's next? Rees and Anderson are eager to look into data released along with the CDC's 2011 Youth Risk Behavior Survey -- the sort of numbers represented by the Colorado-centric graphic above. "The CDC numbers are about as official as you can get," he says.

Not that anti-marijuana types won't find some reason to dismiss them. Here's the latest study:

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More from our Marijuana archive: "Marijuana: Ken Buck says Amendment 64 backers care more about profit than people."