The risks of specific activities change over time (driving in the US is now generally much safer than half a century ago, but after 50 years of driving your skills might have deteriorated and you pose a greater risk to yourself and others when you get behind the wheel). And if you want to know if intercontinental flying (which you might do infrequently) is riskier than downhill skiing (which you may have done for many years), you must have a rather accurate comparative yardstick. And how does one compare the risks experienced in different nations—say, driving in the US, being struck by
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