That's how Duncan Black sees it, anyway. He links to a report by National Public Radio's ombudsman that found experts from right-leaning think tanks were quoted significantly more frequently than scholars from tanks on the left in 2005: 239 to 141. And as Black notes, those 141 come from Brookings and CSIS, which aren't terribly far left.
First of all, I've got a few questions about this count. Are CSIS and Brookings really the only left-leaning tanks that provide experts for NPR stories? Let's try Googling NPR for the New America Foundation. Whoop, there's a bunch of hits. How about the Economic Policy Institute? Oh, gosh, there's a bunch more. How about the Institute for Policy Studies? Hits galore! And those are just the first three leftish tanks that came to mind; I hear there might be a couple more out there. To say the ombudsman pulled his numbers out of his ass would be an insult to asses everywhere.
Second, I notice that Duncan quotes the numerical result, but omits this bit, which immediately follows it:
There may be other experts who are interviewed on NPR who present a liberal perspective. But they tend not be based in universities and colleges and are not part of the think tank culture. That seems to be where most conservative thinking on the issues of the day can be most easily found.
That's putting it mildly. Even if the count were accurate, it wouldn't be a super useful comparison. If, as the conventional wisdom has it, the right is dominant among the think tanks, it's precisely because the right's think-tank network was deliberately set up as a kind of alternative-academia for conservative and libertarian scholars who perceived major universities as unwelcoming. If you're a reporter looking for a quote from some credentialed scholar, you're going to have an easier time finding a professor to offer a view from the left; you're more likely to go to a think tank when you're looking for a contrasting view from the right. So what, exactly, is this supposed to prove again?