How We Misinterpret Medical Statistics

If I told you that reported chlamydia infections in Australia had risen eightfold over a ten year period and that the majority of infections were in people under 29, you might conclude that those pesky youngsters are routinely indulging in unsafe sex. You’d be wrong — a reminder that you need to consider the full context of statistics before drawing conclusions.

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So this is the data we have. Reported cases of chlamydia in 2011 in Australia were around 80,000, and 66,000 of those were in the 16-29 age group. In 1999, the total number for everybody was just above 10,000.

What is driving those numbers up, however, is not necessarily increased infection, but increased detection. “While chlamydia numbers are increasing, these increases are largely because we’re doing more testing, ” associate professsor Jane Hocking, an epidemiologist at the Centre for Women’s Health, Gender and Society at the University of Melbourne, explained in a recent media briefing. “The more you test, the more infections you’re going to find. And the number of tests have also been steadily increasing each year.” Young people are more likely to be sexually active and seeking tests. Unless everyone gets tested, we can’t conclude that they’re more likely to transmit infections than other age groups.

Statistics are useful tools for understanding the world, but they always need to be examined carefully so you understand what’s being said.

That doesn’t mean there isn’t a problem to be addressed. Hocking’s own research suggests that around 5.2 per cent of men and 4.3 per cent of women in Australia have chlamydia. If applied across the relevant population, that suggests 210,000 people should have been diagnosed — a big gap from the 80,000 identified.

As 85 per cent of people with a chlamydia infection show no symptoms, many of them won’t get tested. To actually reduce levels of infection, even more testing is needed, so people who do have the infection can be treated and avoid passing it on to others. “We need to get testing rates up to about 40 per cent before we can expect to see a decline,” Hocking said.

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