By Arnold Kling
As a teacher of high school statistics, I found the controversy over exit polling to be somewhat amusing.
Exit polling is necessarily stratified. You have to plan ahead of time which precincts to sample and how many voters in each precinct to sample. This plan produces a stratified sample by definition.
To use an exit poll to predict the outcome of an election, you need to adjust the weights on your stratified sample…the only information that an exit poll can provide is an estimate of the percent of people in each precinct who voted for each candidate. The exit poll cannot tell you how many voters there were in each precinct.
After the polls close, if the election officials tell you how many people voted in each precinct, then you can use exit polls to accurately predict the outcome of the election.
The essay includes an interactive illustration of the uselessness of exit poll results without weighting information.
What I suspect happened is that unweighted results were released, which is total garbage. Imagine if you surveyed 50 voters each from ten precincts and released the results of that survey, notwithstanding the fact that some precincts had ten times as many voters as others. You could have a totally unbiased exit poll with zero sampling error, but until the stratification is accounted for, you have no information.
For Discussion. If you’re not a statistician or a pollster, could it actually be a good thing for the public to have a myth that exit polling is erroneous?