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GUIDE: Three expert tips on reading and reporting numbers accurately

For scientists, numbers are the bare bones of many research projects. But for the rest of us, a long list of numbers can obscure the information we need. Misreported or inaccurate numbers make the problem worse.

How can journalists communicate numbers accurately? And how can readers spot misleading statistics and understand the information behind the numbers?

We asked experts about what matters most when examining a number and sharing it publicly.

Report how the number was calculated

Prof David Spiegelhalter, chair of the Winton Centre for Risk and Evidence Communication and author of The Art of Statistics, told Africa Check it’s important that journalists do more than directly quote a number.

“You can’t know whether a number is accurate just by looking at it,” he said. “You need to know how it was derived and to check against other sources.”

Sibusiso Biyela, a science communicator at Science Link, said it’s “suspicious if it’s not shown exactly how a number was arrived at”.

If you can’t tell who calculated a number or how, you need to look for more context before sharing it. Biyela’s advice to journalists is to make how a number was worked out “part of the story”.

How is hunger measured in South Africa?

Data from Statistics South Africa, the country’s national statistical agency, shows that 11.1% of people were vulnerable to hunger in 2019. But how was this determined? 

People are asked whether anyone in their household went hungry in the previous year because there wasn’t enough food. A person is considered to be experiencing hunger if they answer “sometimes”, “often” or “always”.  This kind of information helps a reader understand the statistic. 

Numbers can represent a ‘range of possible truths’

“It is becoming increasingly common for scientists to state the confidence in their evidence,” Spiegelhalter told us. They often do this by providing an “uncertainty” or “margin of error” along with any calculations.

“As a general rule, most numbers that we encounter in reading news about science represent some kind of estimate or range of possible truths,” Siri Carpenter, editor in chief of the Open Notebook, a resource for journalists writing about science topics, told Africa Check. An uncertainty tells us how large that range is expected to be.

Does a large amount of uncertainty about a number mean it can’t be trusted? Surprisingly, the opposite can be true. Spiegelhalter explained that while a large margin of error might make a statistic less useful, “admitting a lot of uncertainty” could indicate that a source was trustworthy.

If a number is reported with absolute confidence and no explanation of why it should be trusted, it has probably been oversimplified. “Words like ‘miracle,’ ‘game-changer,’ ‘cure,’ and ‘breakthrough’ should give anyone pause,” Carpenter said.

So how should journalists report numbers that come with a margin of error?

Carpenter said some stories called for a detailed breakdown of how a study was done, and what its limitations were. Others might require “a sentence or two that reminds readers about the most significant reasons for remaining uncertainty”.

Uncertainty about malaria numbers in Nigeria 

In 2018 major insecticide brand Mortein claimed that “Nigeria contributes 23% of the global malaria cases”. Experts told Africa Check that determining the exact number of malaria cases in the country was almost impossible, partly because of poor surveillance systems and unreliable data. 

At the time the most recent data from the World Health Organization put Nigeria’s share of global malaria cases at 27%, in 2016. But this was based on a statistical model. 

“Each estimate is accompanied by a confidence interval, which indicates how confident we are the estimates are correct,” the WHO told Africa Check. 

“The figures of the number of cases we’ve attributed to Nigeria are the best estimate that can be made, given the data available to feed into the statistical models.”

Round numbers with care

“Rounding means making a number simpler but keeping its value close to what it was,” explains Maths is Fun, an online learning platform. “The result is less accurate, but easier to use.”

The site explains that “73 rounded to the nearest ten is 70, because 73 is closer to 70 than to 80. But 76 goes up to 80.” There are many online calculators that can help you make these calculations.

But Biyela said rounding should be used with care, especially when numbers were being compared. “There are contexts where a few percentage points can make the world of difference.” 

Biyela gave the example of a value increasing from 10.3% to 87%. To compare the numbers, it would be acceptable to round the first down to 10%. This returns an increase of 77 percentage points, not that significantly different to the actual increase of 76.7 points. 

But if the increase was from 10.3% to 10.6%, it would be misleading to round the numbers to 10% and 11%. Without rounding, the difference would be 0.3 points. The rounded numbers would give an inaccurate and much larger increase of 1.  

The Poynter journalism institute gives similar advice on rounding. It says that percentages and large numbers can be rounded off “if this will not harm the accuracy of what you are reporting”. But it recommends more precision with smaller numbers, and that it’s made clear if and when numbers are rounded off.

Readers should also be told if a number’s accuracy has been reduced by rounding. Poynter suggests using words like “about,” “nearly” or “just over” when the figures are not exact.

And it’s good practice to give both the rounded and raw numbers so readers can check your work. “If it’s possible, do both,” Biyela said.

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