"HHS finds heavy marijuana use soaring among young people," the press release from Project SAM, the nation's leading anti-marijuana legalization group, said. "Today, the Department of Health and Human Services found that heavy marijuana use among monthly users – defined as 20 or more days of marijuana use per month – significantly increased among 12-to-17 year-olds in 2014 compared to 2013."

Alarming findings indeed -- but untrue.

Here are the actual numbers (highlighted below), which appear in data from the latest National Survey on Drug Use and Health, which just came out this week. In 2013, roughly 451,000 teens smoked marijuana 20 or more days per month. In 2014, that number dropped to 400,000, according to the survey's estimates. That number is, in fact, the lowest it's been since at least 2009.

That drop is not statistically significant, according to the survey. In other words, the number of kids smoking 20-plus days per month is essentially flat year-over-year, and has been for awhile. So how did Project SAM go from that to "heavy marijuana use soaring among young people?" It turns out they were looking at the wrong part of the report.

When I asked Kevin Sabet, the group's director, how they arrived at that conclusion, he pointed me to a different set of tables from the report — the tables showing the error estimates around the survey numbers. It appeared that Project SAM's number crunchers had mistaken the survey's margins of error for the actual survey results.

When I pointed this out to him, Sabet thanked me, and the group issued a corrected press release Friday evening. The original headline, which was unsupported by the data, remained up until Saturday afternoon.

"We made an error and issued a correction immediately," Sabet said. "We're human. What's not in error is that overall marijuana use is up, and perception of harm is down ... and daily college use is at a 30-year high. But that seems to be ignored."

It seems clear that this was an honest mistake, and that Project SAM wasn't deliberately attempting to mislead. But their misstep comes after other deeply questionable assertions and inaccuracies pushed by anti-drug organizations going back to the dawn of the War on Drugs. Just this year, anti-drug group DARE published a story on its Web site claiming that marijuana edibles, "sold on the street as ‘Uncle Tweety’s Chewy Flipper’ and ‘Gummy Satans’," were responsible for over 20 deaths. The story was actually an elaborate joke originally published on a satire Web site.

There's no doubt that sussing out the effects of marijuana legalization is really, really hard. New scientific studies come out nearly every week, with sometimes conflicting results. The same numbers might get spun in totally different directions by pro- and anti-legalization advocacy groups.

But getting these facts right matters now, perhaps more than ever, as a number of states will consider whether to join Colorado, Washington, Alaska, Oregon and the District in legalizing marijuana in 2016.

Update: This article has been updated to reflect the headline change on Project SAM's corrected press release.