Over the weekend, Mark Blumenthal of the Huffington Post published a lengthy review of the Gallup poll’s methodology. It is a technical read, but I encourage you to give it a careful look.

Blumenthal’s bottom line is that Gallup tends to place the president’s job approval rating about 2.5 points below the average of similar polls, and that a portion of this can be chalked up to under-sampling non-whites.