Princeton Election Consortium

OUR PROCESS

The first step is to calculate the probability of winning a state, taking into account the variability of polls. This is done by calculating simple statistics on the polls: median and estimated standard error of the mean (SEM). The median is used as an outlier-resistant method of estimating the mean. A floor on the effective SEM is used to account for intrinsic sampling error and inter-pollster variability. The median and effective SEM are then converted to a z-score and probability of a win using the normal distribution (bell-shaped curve). The last 3 polls are used, or all polls with a median (changed on 7/14/2016) an end date within 7 days of the most recent poll are used, whichever is greater. Occasionally a tie can result in the use of 4 polls. At present, it is permitted for a polling organization to be the source of more than one poll for a state. From these statistics, a win probability is calculated for each state using the t-distribution. A t-distribution is used instead of the normal distribution because the t-distribution has longer tails, providing a way to allow for surprising outlier events.