In 2015, an alarming telegram from the Danish news agency Ritzau was shared by several news outlets.

The story was about a study that found more brain tumours among women who use birth control pills or other hormonal contraception than among women who do not.

The risk of brain tumours is 90 percent higher among birth control pill users, the study concluded. Wouldn’t that make some women consider dropping the birth control pills?

Only further into the article, the reader was told that the risk of getting a cancerous tumour in the brain was in fact extremely low. Only 5 out of 100,000 women of childbearing age are affected per year. That is, the risk of getting a brain tumour is already only 0.01 percent.

If the risk for women on birth control pills increases by 90 percent, it increases to 0.019 percent — still a very low risk. But the Ritzau story did not reveal that.

In fact, so few Danish women get brain cancer that the data set is not large enough to assess whether birth control pills actually increase the risk, we wrote on Videnskab.dk. A more balanced version of the story also made its way to Time and The Guardian.

Not uncommon to exaggerate

The brain tumour story is an example of misinformation about the risks of a drug that thousands of women use.

But it is not uncommon for journalists to exaggerate risks and effects when throwing themselves at research based on numbers and statistics.

The blunder usually occurs because the journalist confuses relative numbers with absolute numbers.

Relative numbers (or values) are dependent on other numbers. They typically show an increase or a decrease in percentage.

In the example of birth control pills and brain tumours, the relative risk increase is 90 percent. The number indicates how big the difference is between the number of brain tumours in women who take birth control pills and women who do not.

Great potential risks

Absolute numbers (or values) indicate how great the risk of brain tumours in birth control pill users is when you take into account how few women actually get brain cancer.

Contrary to the relative number, the absolute number gives the individual contraceptive pill user an impression of how low her risk of getting a brain tumour is.

You can create unnecessary fear and give a misleading picture of how high risk an effect really is if you state only the relative numbers.

Another example: you report in a news story that the risk of a plane crash has increased by 100 percent. But you fail to explain that 100 percent covers a single year in which there was  one plane crash out of 3 million flights, while the next there were two. This provokes unfounded fear of flying.

The absolute figures are not always included in the press release, nor in the scientific article, so ask the scientist for them. Ask what the relative numbers actually cover and how high the risk is for the individual, no matter what topic you are writing about.

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