How often in your life have you heard a political commentator say something like “Well 50% of Americans may disapprove of the job the President is doing, but he is still better off than Members of Congress, of whom 70% of Americans disapprove”?

Countless op-eds, essays and news stories travel the same lines. Typically, they try to forecast elections by analyzing presidential approval ratings and Congressional approval ratings (or approval ratings of one of the parties).

But approval ratings of groups of politicians can’t be interpreted in the same fashion as approval ratings of individual politicians, particularly if we are trying to guess what will happen in an election. At least three flies trod the ointment:

(1) Everyone who responds to a poll about Presidential approval is expressing an opinion about the same person. But poll respondents who express approval or disapproval of a large group of people (e.g., Congress or the Democratic Party) could be giving an opinion about different individuals or subgroups within that greater whole. Their opinions therefore can’t be reasonably aggregated as if they had the same meaning. For example, Nancy Pelosi and Harry Reid-loving respondents’ disapproval of “Congress” may refer to how they loathe the Tea Party Caucus whereas Tea Party respondents’ disapproval may reflect how they detest Nancy Pelosi and Harry Reid.

(2) Ever wonder why pre-election polls often show that voters are overwhelmingly hostile to incumbents yet the results of the ensuing elections indicate that those same voters went out and supported an incumbent? Human beings have a self-serving cognitive bias when they make personally relevant judgments. If you ask a smoker “What proportion of people who smoke just like you will get lung cancer?”, and then ask “What is your own chance of getting lung cancer?” the smoker will usually explain why, for some reason or other, their personal risk is lower than what they quoted for the group of people who smoke just like them. The same phenomenon can be at play when someone tells you that 90% of the Congress are bums who should be thrown out of office, but also maintains that “Good Old Representative Smith” in their own district happens to be in the 10% of paragons on the Hill. You can’t play this self-serving cognitive game with yourself when a pollster asks you about whether you approve of the President because we all have the same President. If you think my President is a bum, by definition you think yours is too.

(3) Everyone can vote for the President, but no one gets to vote for more than a small slice of “the Congress” or one of the major parties. If you disapprove of the President, you have the power to act on the object of your disapproval when you vote. But even if you loathe most of the Congress and/or one party, you don’t have much power to translate those attitudes into action in your voting. That’s another reason why Presidential approval ratings can’t be interpreted in the same frame as generic party or Congressional approval ratings

How can you compare apples and apples when forecasting elections? Analyze data from those polls that follow questions about approval of the President with queries about approval of the Congressional Representative for the respondent’s own district and each of the individual Senators from the respondent’s home state.

[Cross-posted at The Reality-Based Community]

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Keith Humphreys is a Professor of Psychiatry at Stanford University and served as Senior Policy Advisor in the White House Office of National Drug Control Policy in the Obama Administration. @KeithNHumphreys