Yesterday we talked some about what can cause polling errors. But let's cut to the chase: we really want to know how well today's polls can predict election outcomes in November.
This has an easy answer, for Senate polls at least: on average, the margin of a poll you see today will be wrong by about 7 points, in one direction or another, assuming 2014 is similar to 2012 and 2010. That means some polls will have a lot less error, and some will have a lot more—maybe even 20 points off.
The graph below shows the errors of all the Senate polls from 2012:
You can see there were plenty of polls, even less than ten days before the election, that missed the final margin by 10 points or more. The line is a local regression.
But what about 2010? Peek below the fold to see.