TLDR: Assumption No. 1: The high number of undecided and third-party voters indicates greater uncertainty. ... If we tweaked our model so that it only conside

TLDR: Assumption No. 1: The high number of undecided and third-party voters indicates greater uncertainty. ... If we tweaked our model so that it only considered the number of days left until the election when calculating uncertainty, Trump’s chances would decline to 10 percent, while Clinton’s would rise to 90 percent. Assumption No. 2: The FiveThirtyEight model is calibrated based on general elections since 1972. ... If we calibrated the model based on presidential elections since 2000 only — which have featured largely accurate polling — Clinton’s chances would rise to 95 percent, and Trump’s would fall to 5 percent. Assumption No. 3: The FiveThirtyEight model uses a t-distribution with “fat tails,” which gives a greater likelihood of rare events. ... If we used normal distributions instead of t-distributions, Clinton’s chances in polls-only would rise to 87 percent from 85 percent, while Trump’s would fall to 13 percent. Assumption No. 4: State outcomes are highly correlated with one another, so polling errors in one state are likely to be replicated in other, similar states. ... If we assumed that states had the same overall error as in the FiveThirtyEight polls-only model but that the error in each state was independent, Clinton’s chances would be … 99.8 percent, and Trump’s chances just 0.2 percent.