Guest post by Andreas Murr, University of Warwick.

On Sunday 26 September Germans will go to the ballot box to cast several votes in state and federal elections.  The Economist and several academics have published their federal election forecasts.   But who do Germans think will win the election?  Two research teams have used such citizen forecasts to predict the upcoming federal election.

Murr and Lewis-Beck predict the parties’ national vote shares based on citizens’ responses to the question “Who will win the general election?”  Their prediction is based on a regression model of historical vote shares on citizen forecasts and whether there was a grand coalition.  The historical data goes back until 1980.  Combing their regression model with a recent Politbarometer survey from June, they predict CDU/CSU to win 34% of the vote and SPD to win 21%. of the vote.  According to them, a CDU/CSU/SPD coalition seems the safest bet statistically, though a CDU/CSU-Greens coalition is not out of the question.

Kayser et al. predict both constituency winners and parties’ national vote shares by simply aggregating citizen forecasts collected in a survey the last two weeks.  They collected two different kinds of citizen forecasts.  First, they asked citizens to forecast which candidate will win in their constituency.  And, second, they asked citizens what vote share each party will win nationally.  The authors then simply predict the constituency to be won by the candidate who most citizens say will win.  And, they predict the vote share of a party to be the average of citizens’ forecasted vote share.  In other words, only survey data and no historical data was used to forecast the election.  Citizens collectively predict that CDU/CSU will win more constituencies than the SPD (174 v. 98).  However, they also collectively predict that SPD will win a higher national vote share than CDU/CSU (25% v. 23%).  The Greens are predicted to win 16%.

Why do the forecasts of the two teams differ?  The two teams use different methods and forecast with different lead times.  However, we can update the forecast of Murr and Lewis-Beck by using the most recent Politbarometer survey from mid-September.  This way the forecasts differ only in method.  If we do this, then both forecasts go in the same direction.  The Murr and Lewis-Beck model then predicts CDU/CSU to win 25% and SPD to win 31%.  In other words, their updated model would also predict the SPD to win a majority of the votes, though a bigger one than predicted by Kayser et al.  This said, Murr and Lewis-Beck did some analysis of the optimal lead time of their model: they find that it forecasts more accurately with a lead time of two months (June) than one month (September) on average.  Of course, soon we will know which lead time or method forecasted better in this election.