Hillary Clinton currently has a 71 percent chance of winning the presidency, according to The Upshot’s forecasting model. This is down from 90 percent last month, but higher than some other models we’re tracking, which put the odds between 58 percent and 85 percent.

Part of the discrepancy comes from the use of different information. The PredictWise number — 74 percent — incorporates a sharp jump in betting markets that occurred during the first presidential debate. This jump, if it’s real, is not yet reflected in polls, which take days to conduct.

But most of the difference has to do with different model assumptions. Poll-based models need to take a position on two key questions: How useful are older polls? And to what extent do states move together?

The answer to the first question informs how heavily the model weights new information. Weighting new polls very heavily means you can be quicker to respond to new trends and to pick up turning points, but it also makes your forecast more unstable and more likely to react to noise (what we like to call “chasing shiny objects”). Ideally a forecast finds a balance between these two extremes, trading some quickness for stability.