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Expert evidence and forensic science in the courtroom
Expert evidence and forensic science in the courtroom

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5 Numbers

Both lawyers and laypeople often struggle with numbers. The Latin phrase ‘iudex non calculat’ (a judge does not calculate) means that it is the quality and not the quantity of arguments that counts – but some lawyers jokingly take it to mean that judges are not good at maths. While there is some truth that lawyers and laypeople can be uncomfortable with the numbers often used in expert evidence, there are ways of overcoming this problem.

In Box 4, you will be reminded about probabilities, a mathematical idea that underpins much of expert evidence.

Box 4 About probabilities

Much expert evidence is discussed in terms of numerical probabilities. These can be expressed as a percentage where 0 per cent is impossible and 100 per cent is a certainty (see Figure 13), or as a number where 0 is impossible and 1 is a certainty.

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Figure 13 The scale of probability
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A diagram that shows the probability of different events. It shows a scale from impossible on the left to certain on the right. Events are labelled above the scale, and the corresponding percentage probability is shown below the scale. The probabilities and events shown are: the probability that a clover has four leaves is 0.01 per cent; the probability of getting struck by lightning is 0.0033 per cent; the probability a person is left-handed is 10 per cent; the probability that it will rain on a given day in the UK is 33 per cent; the probability of a fair coin toss landing on tails is 50 per cent; the probability that a baby will be a boy is 51 per cent; the probability of surviving an aeroplane crash is 95 per cent.

Figure 13 The scale of probability