Replication data for: German election forecasting: Comparing and combining methods for 2013

The present study reviews the accuracy of four methods for forecasting the 2013 German election: polls, prediction markets, expert judgment, and quantitative models. On average, across the two months prior to the election, polls were most accurate, with a mean absolute error of 1.4 percentage points, followed by quantitative models (1.6), expert judgment (2.1), and prediction markets (2.3). In addition, the study provides new evidence for the benefits of combining forecasts. Averaging all available forecasts within and across the four methods provided more accurate predictions than the typical component forecast. The error reductions achieved through combining forecasts ranged from 5% (compared to polls) to 41% (compared to prediction markets). The results conform to prior research on US presidential elections, which showed that combining is one of the most effective methods to generating accurate election forecasts.