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4 Tips for Avoiding Math Errors When Reporting

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This post was originally published by The Journalist’s Resource, a publication of Harvard Kennedy School’s Shorenstein Center on Media, Politics and Public Policy, and is reprinted with permission.

When you’re reporting on changes in numbers such as state funding, crime rates, and opinion poll results, it’s important to tell audiences how much a number has risen or fallen over time. Before you can do that, though, you need to know the difference between “percent change” and “percentage-point change.”

“Percentage change and percentage-point change get mixed up all the time,” says Jennifer LaFleur, a senior editor at The Center for Public Integrity who has taught classes and workshops on using statistics in news stories for more than a decade. She also teaches data journalism at American University. Here are four tips to help journalists master these key mathematical concepts.

1. Keep in mind the “percent change” is the rate of change. Use “percentage point” to indicate the amount of the change.

We use “percent” to describe how much a number has changed in relation to a previous number.

As an example, let’s say state legislators set aside $30 million to fund a new traffic safety program in 2021. This year, they budgeted $38 million.

Based on that information, we can say funding for the traffic safety program increased almost 27% from 2021 to 2022 ([$8 million divided by $30 million] x 100).

While you can do the math yourself with these step-by-step instructions, it’s easy to find a percentage change calculator online.

When comparing percentages, use “percentage points” to describe the difference between them. For instance, there’s a 27 percentage-point difference between 17% and 44%.

It’s worth noting that Associated Press style calls for using the symbol for “percent” in news stories but writing out “percentage point.”

2. Be extra cautious when comparing percentages, because this is where many people make mistakes.

When you add or subtract percentages — for example, when you subtract the percentage of children diagnosed with autism in 1981 from the percentage diagnosed in 2021 — describe the result using percentage points.

“If you’re comparing two percentages, it’s percentage points,” says LaFleur.

If a local county health department reports that its percentage of children diagnosed with autism increased from 1% in 1981 to 5% in 2021, it’s accurate to say the diagnosis rate rose 4 percentage points over that period.

The rate did not rise 4%. In this scenario, the diagnosis rate jumped 400%, or five-fold.

Because this type of error is common, LaFleur encourages journalists to double check percentage-change figures in government reports and other documents — including news stories from journalists at the same news outlet.

“Part of the problem is, oftentimes, there’s not someone in the newsroom who’s spotting [these types of errors],” she explains. “A reporter might have done it for years and not realized what they were saying is wrong.”

3. When covering a story based heavily on numbers, look for ways to eliminate some numbers without sacrificing meaning or context. For instance, instead of reporting that tuition at a local college increased 100%, say it doubled.

Because number-heavy stories can confuse and overwhelm audiences — and some people avoid them — choose carefully which numbers you want to highlight. Both the Poynter Institute and the Donald W. Reynolds Center for Business Journalism recommend no more than three numbers per paragraph.

LaFleur suggests replacing some numbers with words to help audiences grasp information quickly. Many people will immediately know what you mean when you say a number doubled, tripled or grew four-fold. Because it’s less common to say a number rose by 100%, 200% or more, some people may need to stop and do some math to figure out what that means.

“I always just advise people to keep it in as simple terms as possible,” LaFleur says.

4. If your math skills need sharpening, get training. A firm understanding of key math concepts helps journalists spot misinformation and math errors.

Journalism and higher education organizations offer a variety of online courses, in-person trainings and other resources to help reporters and editors brush up on their math skills and build new ones. Here are some:

  • The Poynter Institute offers the “Math for Journalists Certificate,” a four-hour, online course that costs $29.95. Participants earn a certificate upon completion.
  • Journalists can sign up for free courses in data and numerical literacy on the online learning platforms Coursera and edX.
  • Investigative Reporters and Editors (IRE) typically hosts workshops focusing on statistical literacy during its annual conferences. At IRE’s annual data journalism conference in 2021, LaFleur and colleague Jaimi Dowdell of Reuters led the session, “How to Approach Stats In Your Story.”

Temple University mathematician John Allen Paulos underscores the importance of mathematical literacy in a column he wrote for ABC News in 2004. He explains how a mathematical error in news coverage about a plan to partially privatize Social Security minimized the plan’s impact.

“Usually in print and almost always on television, the proposal is said to call for 2% of Social Security taxes to be diverted into private accounts,” he writes. “Looking a little further, however, one can find a few stories noting that the 6.2% of the average American’s taxable income that goes to Social Security taxes will be cut to 4.2%. That’s a 2 percentage point cut — not a 2% cut, but a 32% cut! This will leave a huge hole in Social Security revenues for present retirees.”

Paulos later wrote the book, “A Mathematician Reads the Newspaper,” published in 2013. In it, he examines news coverage, pointing out logical fallacies and other ways numbers can mislead.

Other books worth checking out

Additional Resources

The Rise of Science-Based Investigative Journalism

Eight Simple Ways to Let the Spreadsheet Do the Math So You Can Focus on the Story

Interpreting Data: Tips to Make Sure You Know How to Read the Numbers

Denise-Mary Ordway joined The Journalist’s Resource in 2015 after working as a reporter for newspapers and radio stations in the US and Central America, including the Orlando Sentinel and Philadelphia Inquirer. Her work also has appeared in publications such as USA TODAY, the New York Times, and Washington Post. 

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