Elections scholar Christopher Wlezien very kindly sent me the data that he and Robert Erikson used to construct the graphs in The Timeline Of Presidential Elections: 1952-2008. Adding in 2012 data, I took time series from 16 Presidential campaigns and calculated the standard deviation of the total movement as a function of time. This is a measure of uncertainty about November based on polls for a given day. This graph shows the ±1 standard deviation interval in red:
This year, January 1st was 312 days before the election. At earlier dates, the standard deviation is between 14 and 22 percentage points. You can see the variation across 16 Presidential campaigns in the gray traces. So polls before the new year really are quite uninformative.
Now look at later dates: the gray curves converge. Consequently, the standard deviation declines, and reaches a local minimum at 270 days before the election, in mid-February- close to the start of primary season. So before the primaries start, February is a time when national polls tell us a fair amount about the final outcome.
But wait! After that, the standard deviation creeps upward. The election is 169 days from now, and in about a week the standard deviation hits its maximum value for 2016. Truly, now is the single worst time to be paying attention to fresh polling data. I don’t know why this is. It could be because typically, one or both parties are still going through an active nomination contest – as Hillary Clinton and Bernie Sanders are doing now.
Amusingly, national polls won’t reach their February levels of accuracy until August.