I couldn't help but have a go at the claim that Rachel Riley was, for a time, the youngest person in the world (The Actuary, August 2015).
While global data is a bit shaky, the World Development Report 1986 can be used to find an estimate for 166.7 million births worldwide in 1986, giving a birth rate of approximately 5.3 per second. Factoring in an estimate from the US National Library of Medicine of 3% of labours involving multiple births, and plugging this into the Poisson distribution gives an 82.7% chance of two births occurring simultaneously.
We can therefore conclude that the chance of Rachel Riley being the youngest person on the planet at the instant of her birth is about 17%. Even then she would only have been so for an average of 0.2 seconds!
Andrew Kelly 19 August 2015