28/07/2017
AGGREGATE POLL SCORE FOR KENYA'S PRESIDENTIAL ELECTION
There are several methods available for aggregating polls. One can use a simple average of polls to come up with an aggregate share of vote for each candidate or use other more complex methods to achieve the same goal. A simple average is, however, unreliable given the scarcity of data, sample size differences, differences in margins of errors, poll accuracy differences, and pollster biases. We, therefore, adopted a weighted approach employed by Kenneth Bunker and Stefan Bauchowitz in Chile.
The method takes account of each poll’s margin of error, the accuracy of the pollster, and the time at which the poll was fielded. As reported in Table 1, all the four pollsters report both the margins of error and the time the polls were conducted. However, to calculate the accuracy of the pollster, prior information on how accurate a pollster predicted a previous election is important.
We used the 2013 presidential election to come up with the accuracy scores. Among the four firms, only Infotrak and Ipsos conducted polls prior to the 2013 presidential election. The other two firms are, therefore dropped from the aggregation sample for lack of data.
Here are the results in the chart. The table shows how the aggregate score compares with the other pollsters.