Beyond the Margin of Error (6): The Curious Incident of the Young Republican Minorities

In the last post I ended with the phrase 'Beware the crosstabs.' Today I will expand on that.

Here's what happens if you break down the aggregated Daily Kos/SEIU/PPP polling to date by race and age. (For racial groups that are not shown, N is too small for the younger generations.)

Let me be VERY clear here, before I begin discussion, that some of these numbers are not correct.

The first thing you might notice is that the 18-29 age group appears less supportive of Obama among Hispanics and African-Americans than older demographics, although among whites it is more supportive. At first blush, this is concerning. Republicans have said that as minorities move into the middle class they will start to vote more conservatively... could they actually have been right? Are these pampered young middle class kids rebelling against their parents? Or is this a problem with missing cell phones (PPP does not call cell phones)? Is it that Young Republican minorities are much more likely to have a landline? Or something else?

We'll focus on African-Americans because the numbers are more extreme and therefore easier to work with. First, we will eliminate the possibility that these strange numbers are actually correct by simply looking at the generic ballot numbers instead and comparing them to the exit polls for 2010 and 2008:

It takes all of about two seconds to see that the aggregated Daily Kos polling numbers for younger African-Americans are, shall we say, slightly misleading.  There is simply no plausible way that Democrats are currently underperforming 2010 results among some of the most Democratic demographic groups in the country.

But the error bars are so small! How could the numbers be so far off? What we see here is a great example of polling error that is not included in the mathematical margin of error calculation. Follow me below to see what this error is.