Workers more productive if allowed to use the Internet for leisure

The headline (and news hook) does not necessarily follow from the study results. What the study actually found:

“People who do surf the Internet for fun at work - within a reasonable limit of less than 20% of their total time in the office - are more productive by about 9% than those who don’t,” [Dr Brent Coker] says.

So first, they've excluded those (mentioned later in the article) who spend an excessive amount of time leisure surfing.

Second, what if tolerance of leisure surfing is essentially a reward granted to the productive? They studied 300 workers, of whom 70% engaged in leisure surfing, and they were "9%" (metric unclear) more productive . What if the unproductive workers who leisure-surf are promptly let go, and thus not in the 300-person population at all?

That the university where the study occurred does not itself link to the detailed written report -- instead only providing only video and audio of an interview -- is also suspicious.