---
title: "Appendix C: Sensitivity to the turnout forecast"
description: "The candidate preferences of voters and nonvoters in 2014 were very different. This fact makes cutoff methods very sensitive to the chosen turnout threshold. Using the Perry-Gallup method, the forecast margin ranges from a tie vote (47%-47%) with a more inclusive model (a turnout forecast of 60% of registered voters, 42% of the general public) […]"
date: "2016-01-07"
authors:
  - name: "No Author"
url: "https://www.pewresearch.org/methods/2016/01/07/appendix-c-sensitivity-to-the-turnout-forecast/"
categories:
  - "Election 2014"
  - "Election 2016"
  - "Methodological Research"
  - "Political & Civic Engagement"
  - "Politics & Policy"
  - "Survey Methods"
  - "U.S. Elections & Voters"
  - "Voter Demographics"
  - "Voter Participation"
---

# Appendix C: Sensitivity to the turnout forecast

[![Sensitivity of Perry-Gallup index](https://www.pewresearch.org/wp-content/uploads/2016/01/PM_15.01.07_voterModels_sensitivity420px.png)](https://www.pewresearch.org/?attachment_id=276443)

The candidate preferences of voters and nonvoters in 2014 were very different. This fact makes cutoff methods very sensitive to the chosen turnout threshold. Using the Perry-Gallup method, the forecast margin ranges from a tie vote (47%-47%) with a more inclusive model (a turnout forecast of 60% of registered voters, 42% of the general public) to a 5 percentage point Republican advantage with a more restricted model (a forecast of 45% turnout among registered voters, 33% of the general public). Using the vote among verified voters as a yardstick for where voter preferences stood at that time, a turnout forecast of 50% of registered voters (35% of the general public) comes closest to the benchmark, yielding a Republican lead of 3 points (49% to 46%).

Given that [estimates](http://www.electproject.org/2014g) for the 2014 general election put the national turnout at 35.9% of the voting eligible population, it would seem that this is the correct cutoff to use. But it is clear that the survey sample used here – registered voters who completed two waves of interviews and were matched to the voter file – overrepresented likely voters, since 63% of them (not 50%) are verified as having voted. As a result, a 60% turnout cutoff for registered voters (42% of the general population) was used for analysis.

Most survey samples are likely to have a similar bias, if not to the same extent. The problem is that it is difficult to know how much a given sample overrepresents likely voters. For this sample in this election, the Perry-Gallup method produces a forecast that is too Democratic when the appropriate cutoff is employed.