Numbers get a bad press. Almost alone of the academic disciplines, mathematics is one where expressions of ignorance are more of a boast than a shameful admission (imagine admitting at a dinner party that you can't read). Yet numbers are more important than ever. They are the language of most of science and much of government, two forces that do much to shape people's lives. They are the nervous system of any modern country, marshalled in support of arguments over everything from defence to which diseases should be treated.

Happily for the number—shy, help is at hand. A book about numbers and how to interpret them doesn't sound like interesting bedtime reading. Yet in the hands of Michael Blastland and Andrew Dilnot, respectively producer and presenter of "More or Less", a BBC radio programme on the subject, that is what it becomes.

The chapters proceed from simple concepts to more complicated ones, starting with the simplest of all; working out how big a number is. That is harder than you might think. The British government's promise to spend £300m ($600m) over five years on child-care looks impressive at first glance. Only after working through the calculations does one realise that it amounts to only £1.15 a week for every family.

The authors avoid a hectoring tone. Their prose is light-hearted and never condescending. Amusing (or occasionally frightening) examples make theoretical arguments instantly accessible. The authors point out that most people have a higher-than-average number of feet (it takes only a single man with one foot to bring the average below two). Incomes are the other way around: thanks to a few billionaires, most people earn less than the average.

Later on, trickier and more emotive subjects are tackled. Much of the second half is taken up with the devilishly tricky business of trying to extract causation from correlation. One such issue is the so-called "cancer clusters". A mobile-phone mast is erected in a village. Soon after, cancer rates rise to several times the national average. But masts are common, and some villages are bound to develop high cancer rates through nothing more sinister than sheer bad luck. Proving correlation is easy, but proving causation—despite how obvious the links may seem—is not. The authors make the sobering point that mortality rates for doctors vary so much that, even if he had been constantly monitored, Harold Shipman (a British doctor who murdered at least 200 patients) would have racked up a body-count of several dozen before coming to official attention.

The central problem, as the authors admit, is that numbers can often be deeply counter-intuitive. Individuals find it difficult to cope with the vast quantities of cash consumed by a modern nation state, and everyday rules of thumb can sometimes lead to utterly wrong conclusions. That is what makes this book so valuable; it provides a reliable guide to a treacherous subject, giving its readers the mental ammunition to make sense of official claims. That it manages to make them laugh at the same time is a rare and welcome feat.