The model is a polls-only forecast: we do not consider any fundamentals of the race including, but not limited to, fundraising, spending, demographics, ad campaigns, or candidate presence in a state. Our polling average weights polls by their sample size, FiveThirtyEight pollster rating, its recency calculated by its end date, and its sampling population (i.e. likely voters, registered voters, all adults). Our model takes the polling average as well as its margin of error at the 95% confidence level to perform a daily-simulated guided random walk to days of the election based on a trend of the historical polling average of each jurisdiction. After each contest, we simulate candidates dropping out as a probabilistic function dependent on their delegate total as well as their estimated polling average nationwide as well as in the next primary contest. The share of votes each remaining candidate receives from any candidates that have withdrawn is calculated via crosstabs from polls which ask for second-choice preferences nationwide. We run 4,000 simulations of the nominating contest daily; the aggregate results of those simulations produce the forecast for that day.
Update (02/03): An earlier version of the model erroneously assigned New Hampshire's final delegate to Nikki Haley instead of Donald Trump (a 12–10 split rather than the correct 13–9). This error did not affect any part of the simulations other than in the total delegate count, which have been retroactively corrected. Total delegate counts starting February 04 reflect the correction made.
Update (02/21): The part of the model which splits total statewide vote into congressional districts has been modified to reduce the variance (congressional vote shares for each candidate will tend to be more similar to statewide vote shares). This change will be implemented starting February 22 but will not retroactively change previous model results. This change will only affect delegate predictions for South Carolina and does not have any bearing on the statewide percent each candidate is projected to win nor their odds of winning the state as a whole. We expect this change to increase Donald Trump's statewide delegate projections in South Carolina.