Or perhaps they rarely quote right-wing think tanks? Still, a bit more googling: 7 hits for "right-wing think tank" vs 4 hits for "left-wing think tank", so a

Or perhaps they rarely quote right-wing think tanks? Still, a bit more googling: 7 hits for "right-wing think tank" vs 4 hits for "left-wing think tank", so apparently they don't use either term very often. 1200 hits for "liberal think tank" vs 285 for "conservative think tank" vs 447 for "libertarian think tank" (all of which seem to be Cato) vs 6 for "moderate think tank". 3 for "Republican think tank", 345 for "Democratic think tank" and 57 for "non-partisan think tank" I take your point, though. What you'd really need to do in order to analyze bias would be to check out "think tank" is used without an adjective and figure out what kind of think tanks those are.