David M Rothschild on Posted on

PredictWise’s market-based predictions for the GOP race in Indiana have been very different than FiveThirtyEight’s poll (and endorsement)-based forecasts. PredictWise has given Donald Trump an increasingly large lead over any/all rivals since well before the New York primary on April 26. FiveThirtyEight gave a growing edge to Ted Cruz, until suddenly flipping today, Sunday May 1.

PredictWise’s prediction is an aggregation and debiasing of prediction-market data only. FiveThirtyEight is using a mix of polling data and endorsements. The details are not clear, but they are presumably ran a model based on historical primary-by-primary polling and endorsements, by day, and the Polls-plus forecast reflects that model.

We will never know the true underlying value of probability of victory for Trump on April 30, but it seems like an aggressive model that would have Trump, dominating both the national and Indiana polls, down to Cruz on the strength of endorsements. First, Governor Pence’s endorsement was over the radio, lukewarm, and praised Trump. Second, is Pence nearly as popular with voters as Bobby Knight? He endorsed Trump in person and quite forcefully!