This wasn’t based on one study. It was based to 35 studies with 4804 references.
The Lancet study was reported in this FOX News article
Study: Even Infrequent Use of Marijuana Increases Risk of Psychosis by 40 Percent
http://www.foxnews.com/printer_friendly_story/0,3566,291043,00.html
Most potheads wont believe any marijuana studies that point out its harm unless maybe its being conducted by their drug dealer and thats even doubtful.
According to the article the research was paid for by the British Health Department. That is immediately one strike. NHS will try to do anything to lower their Gubmint paid health care costs.
I’d have to read the whole thing, but from the tone of the talking points, quoting a “40% increase”, it sounds like they just got a 1.40 relative risk and are pimping that.
Any study with a RR < 2.0 is a null study.