Lucky numbers? The secrets of serendipity

Luck is often framed in terms of things that happen in our livesbut perhaps we should feel most fortunate for the fact we're here at all

- David Spiegelhalter

At around midday on 19 August 1949, wreathed in thick mist, a British European Airways DC-3 going from Belfast to Manchester flew into a hillside on Saddleworth Moor in the Peak District, near Oldham. All the crew and 21 of the 29 passengers died on impact or soon afterwards. Eight passengers survived, including a young boy and his parents, although, devastatingly, their younger child was one of the fatalities. That surviving boy became my friend and statistical colleague, Prof Stephen Evans.

I think we would agree that Stephen was lucky. But what do we mean by "luck"? We might say that someone has been lucky, or unlucky, if they have benefited or been harmed by something that was unpredictable and beyond their control. Luck has been called "the operation of chance, taken personally".

Luck comes in three main flavours. Philosophers have identified "circumstantial luck", meaning being in the right place at the right time, or the wrong place at the wrong time - such as Stephen's family taking that particular flight. Then there's "resultant luck", where in a particular situation some people have good and some have bad outcomes due to factors beyond their control. Stephen had the good resultant luck of surviving.

But perhaps the most important is "constitutive luck", which covers all the fortunate or unfortunate circumstances of your very existence; the period of history in which you were born, your parents, background, genes and character traits. So where was Stephen's constitutive luck? He told me that his father's experiences in the RAF led him to insist that the family sat at the back of the plane- and the only survivors were seated at the back. He had the right parents.