Understanding the Margin of Error
By Denise-Marie Ordway for The Journalist’s Resource
Journalists often make mistakes when reporting on data such as opinion poll results, federal jobs reports and census surveys because they don’t quite understand — or they ignore — the data’s margin of error.
Data collected from a sample of the population will never perfectly represent the population as a whole. The margin of error, which depends primarily on sample size, is a measure of how precise the estimate is. The margin of error for an opinion poll indicates how close the match is likely to be between the responses of the people in the poll and those of the population as a whole.
To help journalists understand margin of error and how to correctly interpret data from polls and surveys, we’ve put together a list of seven tips, including clarifying examples.
- Look for the margin of error — and report it. It tells you and your audience how much the results can vary.
Reputable researchers always report margins of error along with their results. This information is important for your audience to know.
Let’s say that 44 percent of the 1,200 U.S. adults who responded to a poll about marijuana legalization said they support legalization. Let’s also say the margin of error for the results is +/- 3 percentage points. The margin of error tells us there’s a high probability that nationwide support for marijuana legalization falls between 41 percent and 47 percent.
Remember that the larger the margin of error, the greater the likelihood the survey estimate will be inaccurate.
Assuming that a survey was otherwise conducted properly, the larger the size of a sample, the more accurate the poll estimates are likely to be. As the sample size grows, the margin of error shrinks. Conversely, smaller samples have larger margins of error.
The margin of error for a reliable sample of 200 people is +/- 7.1 percent. For a sample of 4,000 people, it’s 1.6 percent. Many polls rely on samples of around 1,200 to 1,500 people, which have margins of error of approximately +/- 3 percent.
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