If you’re following my poll tracking and then view Nate Silver’s polls online, the percentages may seem off or contradictory. For example, I may post a poll result which shows Hillary Clinton leading Bernie Sanders by 9 points while a poll on the same day from Silver may show a lead of 15 points. This can be confusing.There’s no contradiction. However, there are different types of methodology in use for the Primaries. The info below is from Nate Silver’s site. Hopefully, it will help make sense of the numbers.
Nate Silver is using two types of forecasts for the Primaries: Polls-Only and Polls-Plus.
The polls-only model is based only on polls from a particular state; for example, only polls of New Hampshire are used in the New Hampshire forecast.
The polls-plus model is based on state polls, national polls and endorsements. The polls-plus model also seeks to account for how the projected results in Iowa could affect the results in New Hampshire and how the results in those states could affect the results in subsequent contests.
Nate Silver Also Uses Polling Averages
Prior to making a “Forecast” (Polls-Only or Polls-Plus), Silver will predict the odds by using Polling Averages.
If polling data is sparse in a state this year, or if voting is still a long way away there, we won’t run a forecast. But we may list a polling average, which you can think of as a FiveThirtyEight version of the polling averages published at RealClearPolitics and Huffington Post Pollster.
For a detailed explanation of Silver’s methodology, visit his website.