In the build-up to the 2019 national and provincial elections there has been much focus on polling results and their predictive value. With the two biggest pollsters reporting such divergent numbers it’s easy to dismiss the whole industry as a waste of time. Regardless of where you stand on polling, most people would agree on a hierarchy of usefulness when it comes to the numbers. The best predictors of future election results are previous elections, and general elections are more useful than by-elections, which in turn are more useful than opinion polls. The disagreements are in the details; just how much less predictive are by-elections when compared with general elections? The greatest strength of general elections is that they are near-universal; they cover every corner of SA at the same time. Opinion polls sample a much smaller segment of voters, typically between 1,500 and 3,000 people at a time. By-elections, over time, can cover the length and breadth of the country but not...

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