Lots of studies (usually government funded) claim to have found similar results. The truth is, potency has gone up noticeably because of selective breeding and improved growing techniques but it's not quite that extreme. When they do these studies they usually test weed that has been kept as evidence since the 60s, and during the past 40 years the THC has degraded and it appears to be less potent than it originally was. And it all depends on where your samples are coming from. Back in the 60s, just like today, there was a wide range of potencies in circulation.