Usually when you see a probability listed at FiveThirtyEight — for example, that Hillary Clinton has a 93 percent chance to win the New Jersey primary — the percentage reflects the output from a statistical model. To be more precise, it’s the output from a computer program that takes inputs (e.g., poll results), runs them through a bunch of computer code, and produces a series of statistics (such as each candidate’s probability of winning and her projected share of the vote), which are then published to our website. The process is, more or less, fully automated: Any time a staffer enters new poll results into our database, the program runs itself and publishes a new set of forecasts.4 There’s a lot of judgment involved when we build the model, but once the campaign begins, we’re just pressing the “go” button and not making judgment calls or tweaking the numbers in individual states.
Anyway, that’s how things usually work at FiveThirtyEight. But it’s not how it worked for those skeptical forecasts about Trump’s chance of becoming the Republican nominee. Despite the lack of a model, we put his chances in percentage terms on a number of occasions. In order of appearance — I may be missing a couple of instances — we put them at 2 percent (in August), 5 percent (in September), 6 percent (in November), around 7 percent (in early December), and 12 percent to 13 percent (in early January). Then, in mid-January, a couple of things swayed us toward a significantly less skeptical position on Trump.
First, it was becoming clearer that Republican “party elites” either didn’t have a plan to stop Trump or had a stupid plan. Also, that was about when we launched our state-by-state forecast models, which showed Trump competitive with Cruz in Iowa and favored in New Hampshire. From that point onward, we were reasonably in line with the consensus view about Trump, although the consensus view shifted around quite a lot. By mid-February, after his win in New Hampshire, we put Trump’s chances of winning the nomination at 45 percent to 50 percent, about where betting markets had him. By late February, after he’d won South Carolina and Nevada, we said, at about the same time as most others, that Trump would “probably be the GOP nominee.”