I am crashing on a deadline for my National Journal column, but want to pass along the analysis just posted by Gallup about the differences in their recent polls on the Democratic nomination. Here is the gist:
Gallup probably does more national polling than any other firm does, so this is certainly not the first time and won't be the last time that one of our polls produces results inconsistent with other polls. When this does happen, though, we do our best to analyze why those differences might have occurred. Unfortunately, these investigations rarely yield a proverbial smoking gun, and, beyond normal sampling error, it is not clear why one poll might differ from other polls -- even in cases when the differences between polls appear to be beyond the margin of error. This appears to be the case with Gallup Poll Daily tracking and USA Today/Gallup poll as well.
Having read my two posts on this subject earlier this week, a pollster friend of mine emailed to ask, "in spirit of Occam’s Razor, am I correct in simply saying 'they don’t have a clue why there is a difference?'"
That's about right.
To their credit, Gallup's analysis thoroughly reviews the subtle methodological differences between the two surveys, and includes a spreadsheet comparing the demographic composition of the two samples. That is more than we can say for some other pollsters. Still, their bottom line is uncertainty:
To reiterate, none of the known differences between Gallup Poll Daily tracking and the USA Today/Gallup poll, based on our analysis, are obvious causes for the disparity in the Democratic ballot estimate between the two samples. In fact, the estimates of the Republican horse race between the two samples were almost identical.
I will let our always throughout readers chew this new analysis over while I go back to drafting my column.