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Tuesday, December 1, 2009

Doing the Math on Obama's Job Approval Rating

A couple of readers have asked about the math in my colleague Jeff Jones' recent analysis of race differences in President Obama's job approval rating. In the story, Jeff points out that Obama's overall approval rating is 49%, while his approval among whites is 39% and among nonwhites is 73%.

The readers have apparently taken the sample sizes for whites and nonwhites given in the "Survey Methods" box at the end of story, multiplied these Ns by the group's respective approval ratings and calculated an average. The results give an average approval rating lower than the actual 49%.

The sample sizes given at the end of the article are the actual unweighted Ns for each group. The reported results, as is the case for almost all national polls done by Gallup and others, are based on weighted results, which take into account sampling variations as compared to U.S. census estimates. Because nonwhites are usually underrepresented in national samples, the final results weighted by U.S. census parameters will give the nonwhites a higher weighted representation and the whites a lower weighted representation. The weighted N for whites is 2,606, and for nonwhites 1,014. Doing the math on these weighted Ns yields the 49% overall approval rating.

1 comments:

Anonymous said...
December 1, 2009 at 11:26 PM  

As the anonymous inquirer to the voracity of your number, I thank you for your response. As I mentioned in my first posting I am not a statistician just a guy with a calculator so your answer (the weighted values) does not immediately make sense to me. Having said that let me make it clear that it is a lack understanding on my part and not a criticism of your answer. I will do a little research but I have every reason to believe your answer is complete and at some level I would guess this becomes proprietary. Again thank you for your response.

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