U.S.News & World Report Tumblelog

We keep seeing the phrase “margin of error,” but what does it actually mean?

Data, uncertainty, and specialization: What journalism can learn from FiveThirtyEight’s election coverage

Expected error — quantified uncertainty — is the price you pay for polling a national sample instead of asking every person in the country how they’re going to vote. It means that small variations in poll numbers are mostly meaningless “noise,” because those last 5.5 percent are effectively down to a coin toss. In other words, you’d expect the very next poll to show the lead reversing about half the time. This 2 percent difference with a 5.5 percent margin of error would never pass standard statistical tests such as the t-test — so you couldn’t publish the result in a scientific paper, a medical board wouldn’t authorize treatment based on such weak evidence, and you certainly wouldn’t want to place a bet.