State polls vs national

Nate Silver of 538 addresses the discrepancies while explaining why his systems are, by his own account, “bullish on Trump”:

Another tricky question is how to reconcile state polls with national polls. For example, there have been no polls of Pennsylvania over the past two weeks, during which time Clinton’s lead has evaporated in national polls (and often also in polls of other states, where we’ve gotten them). The FiveThirtyEight model uses what we call a trend-line adjustment to adjust those those old polls to catch up to the current trend. That’s why our polls-only forecast shows Pennsylvania as a tossup even though Trump has only led one poll there all year. Those older polls came from a time when Clinton led by 5 or 6 or 7 percentage points nationally, and they generally showed her up by about the same margin in Pennsylvania. Now that the national race is almost tied, it’s probably safe to assume that Pennsylvania is very close also. Some of the competing models don’t do this, and we think that’s probably a mistake, since it means their state-by-state forecasts will lag a few weeks behind, even when it’s obvious there’s been a big shift in the race.

Bottom line: Although there are other factors that matter around the margin, our models show better numbers for Trump mostly because they’re more aggressive about detecting trends in polling data. For the past couple of weeks — and this started before the conventions, so it’s not just a convention bounce — there’s been a strong trend away from Clinton and toward Trump.

In other words, as I’ve been saying from the start, it’s too soon to tell anything from the state polls. The fact that the trend is towards Trump is apparent, but it’s not certain that it is the start of a cascade preference that will lead to the predicted Trumpslide.

However, it is the first required step in the process, so that’s a good sign for now.