Probabilities and the polls

As Election Nears, Polls Are Better Predictors

By NATE SILVER, NYT

With fewer than 45 days left in the presidential campaign, it is no longer a cliché to say that every week counts. This is probably the last week, for instance, in which Mitt Romney can reasonably hope that President Obama’s poll numbers will deteriorate because of a deflating convention bounce.

That is not to say that Mr. Obama’s standing — he has a lead of about four percentage points in the polls, on average — could not decline. But if it does, it will probably be forced by Mr. Romney’s campaign or other developments.

So it seems worthwhile to consider what the polling looked like at this stage in previous elections and how it compared with the eventual outcomes.

FiveThirtyEight’s polling database has surveys going back to 1936 (though the data is quite thin through about 1968). Averaging the polls that were conducted 40 to 50 days before the election in each year — the time period we find ourselves in now — several fairly clear themes emerge.

(More here.)