Episode 1: Crime Scene Investigation, forensic science and Bayes' theorem

This week the topic was Bayesian statistics. We interviewed Emma Rixon from Nottingham Trent University about her work as a crime scene investigator and how forensic science uses Bayesian probabilities.

Bayesian probability:

• Introduction to Bayesian Statistics, by computer scientist Kevin Boone.
• A nice simple introduction to conditional probability, from Maths Is Fun.
• Math Goodies, introduction and questions on conditional probability.

Puzzle: I have a bag containing 47 black marble and 53 white marbles. I draw out two marbles. What’s the probability that the second marble I draw out is white, given that the first marble I draw is black?

Solution

The bag contains 47 black marbles and 53 white marbles. This means the probability of drawing a black marble first time is 47/100, but then having done so there are 46 black marbles and 53 white marbles. The probability of drawing a white marble the second time is then 53/99, instead of 53/100, which would be the probability of drawing a white marble the first time.