"A reminder in this political season that we should be careful to classify the political leanings of the think tanks we mention in articles. As noted in 2009, the designations 'liberal,' 'conservative,' and 'libertarian' are appropriate. 'Progressive' isn't. If there is any doubt about what to call a think tank, double check with the Washington bureau."
In 2009, WSJ said it should "endeavor to classify the political leanings" of the think tanks its mentions in articles, to give readers a "reliable signpost." The newspaper noted that the Center for Economic and Policy Research (CEPR), a liberal think tank, had recently been called "progressive" in a recent article. WSJ said the word progressive is "a label some groups prefer because, after all, who doesn't like to be considered progressive?" Here is more of what the WSJ said at that time:
It isn’t always easy to nail down the slant, if any, of individual think tanks. The conservative, liberal and libertarian designations as listed on Wikipedia are fairly reliable, but not infallible. The site also lists “centrist” groups, which we don’t consider a meaningful label, especially because some of those called centrist on the list are openly affiliated with political parties. Nonpartisan, if accurate, is better than “centrist.” If other labeling is elusive, we can at least try to describe the think tank’s financing when it is appropriate: union-funded, or lawyer-funded,etc...
Think Tank watch is currently in the process of making a chart of dozens of think tanks and their political orientation compared to other think tanks.
In the meantime, one decent grouping of think tanks by political ideology can be found in this study by Fairness & Accuracy in Reporting (FAIR). But, according to WSJ standards, many of the ideological groupings would not pass editorial muster. For example, Brookings is listed as "centrist" when in reality, it is left-leaning. And the Cato Institute is listed as "center-right" when a more accurate description is libertarian.
By the way, WSJ is not the only media outlet that is thinking about think tank ideology. For example, in 2011, NPR said it often does a lousy job in identifying the background of think tanks. And inevitably, some think tanks, such as the Peterson Institute for International Economics (PIIE), fight to have their ideology as obfuscated as possible.
Here is an article from 2011 entitled "Calculating The Ideologies of Powerful Think Tanks." Among other things, it contains a list of the 20 most cited think tanks and where they stand on the political spectrum. Here is another list of think tanks by ideology, including Canadian think tanks.
Another interesting question: Is there media bias against certain types of think tanks?