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Oct. 16, 2019, 11:25 a.m. EDT

How to make sense of different polls on impeachment

Who is polled and exactly what they were asked influences the reading on public support

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By Michael Traugott


AFP/Getty Images
A protester outside the Target Center in Minneapolis, Minn., ahead of a rally with President Trump on Oct. 10.

Pollsters are trying their best to track public opinion about the House Democrats’ decision to initiate an impeachment inquiry against President Donald Trump.

Currently, it is doubtful the required two-thirds of the Senate, now controlled by Republicans, would vote to convict Trump. But senators’ votes could change by the time they receive the articles of impeachment from the House.

That is why tracking the changes in the president’s support in the polls is so important. Members of Congress will factor in what the public thinks as well as their own assessments of the president’s behavior.

For a variety of reasons, these are difficult measurements to make. As with all polling, the results depend to some extent on who is polled, what they are asked and when.

Whom are you asking?

New polls on the topic are being released every couple of days, and journalists are reporting on the latest shifts in opinions.

Many stories link attitudes on impeachment to the president’s level of support as measured in his approval ratings.

Attitudes on impeachment — and presidential approval — reflect the current state of political polarization in the U.S. Trump’s approval ratings are determined by strong support among Republicans and disapproval among Democrats; party identification has become the primary determinant of most attitudes in the U.S.

Most polls involving political issues focus on one of three target populations: adults 18 years of age and older, registered voters or likely voters.

Each of these is a subset of the previous one. Most importantly, each subset is generally more Republican-leaning as well.

Presidential approval is commonly measured for all adults. But questions about candidate support — relevant as the U.S. works through the primaries and next year’s election approaches — are typically analyzed for registered or likely voters.

As these samples get smaller by definition and therefore more Republican, this usually mean greater support for Donald Trump and less for impeachment.

Political groups will sometimes produce press releases based on selected samples, such as people “in Republican-held congressional seats and National Republican Congressional Committee targets.” These groups are obviously more likely to be partisan and unrepresentative of the national population.

Partisanship is strongly related to attitudes about impeachment. Consumers should look for differences in opinions by party identification in any single poll. Changing attitudes over time could be related to differences in the partisan composition of samples from different polls, even from the same polling organization.

Democrats are generally more willing to participate in polls while Republicans disproportionately refuse. This potential problem has appeared in some polls in the past. This makes it especially important to compare the partisan makeup of poll samples from different firms, as well as whether the partisan composition is changing over time for the same polling organization.

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