This type of error is not unprecedented — instead it’s rather common. As I mentioned, a similar error occurred in 1994, 1998, 2002, 2006 and 2012. It’s been about as likely as not, historically. That the polls had relatively little bias in a number of recent election years — including 2004, 2008 and 2010 — may have lulled some analysts into a false sense of security about the polls.
Interestingly, this year’s polls were not especially inaccurate. Between gubernatorial and Senate races, the average poll missed the final result by an average of about 5 percentage points — well in line with the recent average. The problem is that almost all of the misses were in the same direction. That reduces the benefit of aggregating or averaging different polls together. It’s crucially important for psephologists to recognize that the error in polls is often correlated. It’s correlated both within states (literally every nonpartisan poll called the Maryland governor’s race wrong, for example) and amongst them (misses often do come in the same direction in most or all close races across the country).
This is something we’ve studied a lot in constructing the FiveThirtyEight model, and it’s something we’ll take another look at before 2016. It may be that pollster “herding” — the tendency of polls to mirror one another’s results rather than being independent — has become a more pronounced problem. Polling aggregators, including FiveThirtyEight, may be contributing to it. A fly-by-night pollster using a dubious methodology can look up the FiveThirtyEight or Upshot or HuffPost Pollster or Real Clear Politics polling consensus and tweak their assumptions so as to match it — but sometimes the polling consensus is wrong.
It’s equally important for polling analysts to recognize that this bias can just as easily run in either direction. It probably isn’t predictable ahead of time.
The key to the issue here seems to be the assumptions that pollsters make before the election: who is going to turn out? Who is most energized? How do we predict who exactly is a likely voter? What percentage of a voting district identifies as Republican, Democrat, or Independent?
One thing that Silver doesn’t address is how this affects both perceptions of and reliance on such political polls. To have a large number of these polls lean in one direction (or lean in Republican directions in previous election cycles) suggests there is more work to do in perfecting such polls. All of this isn’t an exact science yet the numbers seem to matter more than ever; both parties jump on the results to either trumpet their coming success or to try to get their base out to reverse the tide. I’ll be curious to see what innovations are introduced heading into 2016 when the polls matter even more for a presidential race.