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Calculating whether your guilty pleasure pays is hard: statements about health risks and benefits can be tricky to wrap your head around.

With activities like motorcycling or skydiving, one way is to think in micromorts –a micromort being a one-in-a-million chance of dying then and there. This unit is useful for pursuits that could kill you on the spot, says statistician of the University of Cambridge. “When you do these activities, you are going to be healthy unless you are dead.”

For lifestyle choices that chip away at good health – eating to excess, or smoking – think instead in . A microlife is a millionth of a life, and equates to about half an hour. Spiegelhalter reckons that, for regular smokers at least, every two cigarettes costs one microlife. “This enables you to make comparisons across broad ranges of activities using common units, and without having to use very technical units like person years lost per something, or hazard ratios,” he says.

Let’s weigh up skydiving against riding a motorbike. Seven to 10 people die for every million parachute jumps, so that’s 7 to 10 micromorts. On a motorbike, you’d do about 10 kilometres before reaching 1 micromort. So one skydive is like 80 kilometres on a motorbike.

Microlives are useful because of the psychological barrier we put up when considering the long-term health consequences of our actions. “It’s all to do with the end of your life and people, especially young people, tend not to care so much about living an extra year being old and dribbly. So the idea of the microlives is that this is happening to you now. You’re ageing faster because of your behaviour,” says Spiegelhalter.

Good habits can also improve life expectancy. Spiegelhalter has devised a (see “Accounting for taste“) which can help you see whether your good habits outweigh the bad. “The image is of course coming out of the gym and going to the pub,” says Spiegelhalter, “which when I used to go to the gym is what I always used to do. But it’s not to be encouraged.”

Read more: “Guilty pleasures: Which bad habits can you get away with?“

This article appeared in print under the headline “Weighing up the risks”