Statistics 101: Polls and surveys

In previous diaries we’ve looked at some descriptive statistics, and some ways to evaluate statistical arguments. Today, something near and dear to many Kossacks: Polls! We’ll use polls to introduce the ideas of inference, sample, standard error, and confidence intervals. We’ll also look at good and bad ways to ask questions, how to rig polls, and other topics. There’s more, after the fold.

A poll or survey is an attempt to judge the opinion of a population on some question. One class of question is who will win an election. The population is everyone who is eligible to vote, but we are really interested in those who actually vote — if a person has no intention of voting, then it doesn’t matter who that person would vote for. Of course, we can’t ask everyone in the population how they plan to vote — even in the most local election, there are just too many people. It also turns out that it is better to spend money and effort getting the questions right and getting the sampling plan right, rather than trying to get a really large sample. Sample size makes surprisingly little difference, which is why you will see national surveys with sample sizes of only about 1,000 or so. Once we have results from our sample, we use that to make inferences about the whole population — after all, we are not interested in how our small sample will vote (or what they think of an issue) we want to know who…