It is a matter both personal and public. Whether the overall cost of credit on a car or the extent of “waste” in government spending, numbers shape our lives. But how accurate are they, and how fully do we, or our politicians and civil servants, understand these many numbers? Here Blastland and Dilnot, known for their BBC Radio 4 series More or Less, let light into the arcane world of numbers and statistics. When you see stripes among leaves, they remind us, it is only rarely a tiger.

I enjoyed their chapter on averages. Four men are drinking in a bar. Bill Gates walks in, and the drinkers whoop with joy. “Why?” a bemused Gates asks. “Don't you know,” one of the men replies, “what you've just done to our average income?” The distorting effect of averages applies vividly, for example, to what we consider a normal pregnancy and the medical consequences of that view. “First, some mothers give birth prematurely. Second, almost no one is allowed to go more than two weeks beyond the due date ... We count very early births but prevent the very late ones” and so “produce a lower average” than if nature were left alone. Caesareans, arise.

In prose unusually crisp and clear for co-authorship, other chapters address fickle matters such as comparison and correlation. We often accept numbers as we want them to be. This is a clarion call to question far-reaching assumptions. The truth is there. One, two, three ...

The Tiger That Isn't: Seeing Through a World of Numbers by Michael Blastland and Andrew Dilnot
Profile Books, £7.99 Buy the book here

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