Margin of error

My friend, colleague, and former office-mate, Mike Cobb has started a new blog for a local news station (the TV station with the best actual coverage of politics I’ve ever seen) that focuses on polling.  I’m really looking forward to seeing what he does with it.

Here’s his inaugural post where he does a nice job deconstructing recent presidential polls and the generally horrible job the media does in covering them:

Bloomberg recently released a poll indicating President Obama’s approval rating had “rebounded” from 42 percent to 48 percent. They wrote that this increase was “the biggest positive change of his presidency.” The very next day, a Wall Street Journal/NBC poll found Obama’s approval rating was 41 percent. Contrary to Bloomberg, the WSJ wrote, “Obama’s approval rating hits new low.” …

News reports on these polls were misleading, contributing to the appearance of dramatic differences when they are not. This happens because news media usually collaborate with a specific pollster, and their subsequent coverage of their polls ignores other polling on the topic if a different pollster conducted it.

For the Wall Street Journal, the 2 percentage point decline in Obama’s approval rating since January is within the margin of sampling error (plus or minus 3 percentage points), so it is not possible to tell if Obama’s approval rating was declining, or the 2 point change reflected random error introduced by sampling. Further, Obama has polled lower than 41 percent in other surveys, so it’s not accurate to say this was Obama’s lowest rating.

For Bloomberg, ignoring other polls that show a much lower approval rating gives the misleading appearance that Obama’s approval is rebounding – it’s probably improving only within their own polling. To be sure, Bloomberg is not the only poll to find Obama’s approval rating is closer to 50 percent than 40 percent, but a solid majority of all polls suggests approval is currently closer to 40 percent.

I’ve made this point before and it remains a damn good one.  It borders on journalistic malpractice to report your single poll in a vacuum just because it is from a particular polling organization and ignore all the other polls on the same question.  Anyway, the polling average is pretty clear that Obama is in the low 40’s, not exactly where he wants to be and not something that will help in the 2014 elections.

Anyway, nice start from Mike.  I do like the name for the blog, but I would have preferred Marge Innovera.

About Steve Greene
Professor of Political Science at NC State

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