Pregnant woman having ultrasound scan

Are you absolutely sure?

Why risk should be reported responsibly

The way numbers are presented can be misleading, so watch out. In 1995, UK news outlets reported advice from the Committee on Safety of Medicines, which suggested that a new version of the contraceptive pill doubled a woman’s risk of venous thromboembolism (VTE) – a condition in which blood clots form in the veins around the legs.

In the wake of the news, the British Pregnancy Advisory Service estimated that the number of abortions in the UK rose by 13,000, reversing a previously downward trend, as a result of a falling trust in the Pill. The number of births to teenage mothers also increased.

The actual findings relating to the Pill and VTE are as follows: for every 7,000 women that took the previous Pill, one would have VTE. For every 7,000 women that took the new Pill, two would have VTE. The claim of ‘a doubling’ of the risk of VTE was not wrong: the number of women affected had doubled from 1 in 7,000 to 2 in 7,000. This is the relative risk.

But was this the best number to publish? The difference in terms of women affected is just 1 in 7,000 (the difference between 2 in 7,000 and 1 in 7,000 women), or 0.014 per cent. This is the absolute risk.

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About this resource

This resource was first published in ‘Number Crunching’ in June 2013.

Statistics and maths, Health, infection and disease, Medicine
Number Crunching
Education levels:
16–19, Continuing professional development