“One in nine women will get breast cancer.” “One in four people will have a mental health problem.” “One in 11 men will get prostate cancer.” In terms of statistics we know that frequencies are better understood than percentages. But are these the kinds of numbers that we really want to be widely understood?

All these statistics are used as slogans by charities to raise “awareness” and to fight their cause. The “one in four” slogan is also reeled off everywhere from parliament to official NHS guidance, but mental health charities have done a great deal to make this statistic widely known. Yet the data behind it come not from prospective trials but from …