Have you ever noticed that news organizations will pelt you with polling data during an election cycle, and then wonder why the actual results defy the polls? There are a lot of reasons this happens, but one of the most pervasive is due to a misunderstanding about something called the margin of error.
Every survey contains an inherent margin of error that is calculated using methods developed by statisticians. The primary reason for doing this is that survey-takers only talk to a portion of the total population, and they can never be completely sure that those people are representative of the whole group. Some people lie. Some people forget to vote. Others change their minds. And there’s always the possibility that the people you did not ask would have given you completely different answers.
Figuring out the margin of error in a given case requires some mathematical background. Fortunately, it is often done by the pollsters ahead of time, and knowing how to interpret the result is a process that takes less than a minute. It’s so easy to do that any reporter who fails to account for the margin of error is practicing shoddy journalism.
Let’s say that Abel and Bob are running for mayor of Blandeville. The night before the election, a poll of registered voters reveals that 51 percent are in favor of Abel, and 44 percent are in favor of Bob (the rest are undecided), with a margin of error of ± (plus or minus) four percentage points. Abel must be a shoe-in, right?
Not necessarily. The margin of error shows that any of these numbers is likely to be four percent below or above what the pollsters determined. What the poll really says is that Abel’s chances may be as low as 47 percent, or as high as 55 percent. Likewise, Bob’s support may be anywhere from 40 percent to 48 percent.
Since Abel’s lowest possible score (47) is less than Bob’s highest (48), Bob may actually be ahead. Nobody who pays attention to this will be surprised if Bob ekes out a victory tomorrow.
Don’t be fooled by people who ignore a margin of error. It can make all the difference.