August 21, 2016: The forecast has been sharpened using information from this year’s campaign. Also, the Meta-Margin to electoral votes relationship, which generates the November electoral vote “strike zone,” has been updated. See this explanation.
The black curves indicate the median of the Popular Vote Meta-Margin (top graph) and the electoral vote estimator (bottom). For the EV estimator, the gray shaded region indicates the calculated 95% confidence interval. This confidence interval includes sampling error, variation in biases among pollsters, and changes in opinion during the period when the polls were taken. Because pollster biases tend to cancel one another on average, the true 95% confidence interval is smaller, typically less than +/-10 EV.
As of June 2016, the banner at the top of this web page reports both the “random-drift” range of likely outcomes as well as a full Bayesian prediction. The graphs above show the Bayesian probability range. In both cases, the calculation is based on polls only.
The graph’s calculations are explained here. Briefly, the red and yellow zones show a prediction range that combines random drift from current polls with a Bayesian prior. This Bayesian prior is calculated from the assumption that the average Clinton-Trump margin in national polls since January gives the center of the likely range of election outcomes. The prior has a Gaussian range with a sigma of +/-7%, consistent with 1952-2012 but larger than the amount of movement in the 2004-2012 election cycles. In other words, the prior is set to allow anything reasonable to happen.
The red zone is a “strike zone” showing the 68% confidence interval of probable outcomes. It shows the range where the Meta-Margin (or the final Electoral Vote count) is most likely to end up. Approximately two-thirds of the time, polls that are like today’s should end up somewhere in the red band. This band is sometimes called the “one sigma range,” because sigma is quantity resembling standard deviation – a measuring stick that describes how much the Meta-Margin (or Electoral Vote count) will go up or down.
The yellow band shows the range where about 95% of the time, conditions like today’s will end up in the yellow band. It is a “watch zone” that shows a combination of the 95% random-movement confidence interval and the 95% gray-zone confidence interval. The November outcome is nearly certain to be within this range.
Notable dates in 2016:
July 18-21 – Republican National Convention, Cleveland, Ohio.
July 25-28 – Democratic National Convention, Philadelphia, Pennsylvania.
September 26 – First Presidential debate – Hofstra University, Hempstead, NY.
October 4 – Vice-Presidential debate – Longwood University, Farmville, VA.
October 9 – Second Presidential debate – Washington University, St. Louis, MO.
October 19 – Third Presidential debate – University of Nevada, Las Vegas
Notable dates in 2012:
April 25 – Republican National Committee declares Romney the presumptive nominee.
July 12 – Bain Capital / Romney tax attacks begin.
August 11 – Paul Ryan added as GOP VP nominee.
August 27-30 – Republican National Convention.
September 3-6 – Democratic National Convention.
September 17 – Romney “47%” speech emerges.
October 3 – First Obama-Romney debate.
October 11 – Biden-Ryan VP debate. fun!!
October 16 – Second Obama-Romney debate.
October 22 – Third and final Obama-Romney debate.
Notable dates in 2008:
June 7 – Hillary Clinton withdraws from race.
August 1 – McCain campaign releases “Celebrity” ad.
August 25 – First day of Democratic National Convention.
September 3 – Sarah Palin speech at Republican National Convention.
September 11-12 – Sarah Palin on ABC with Charles Gibson; John McCain on “The View.”
September 26 – First Obama-McCain debate.
October 7 – Second Obama-McCain debate.
October 15 -Third and final Obama-McCain debate.