The Daily Mail doesn’t pull its punches: Rise of poisonous caterpillar is unstoppable, say experts.

The caterpillar is the larva of the oak processionary moth, and the story continues with the information that “Each caterpillar is covered in 63,000 hairs which can trigger potentially lethal asthma attacks” – and a warning that the London Olympics might be under threat.

Of course, from a statistical point of view it is the figure of 63,000 that demands attention:

• Do all of these caterpillars have exactly 63,000 hairs? Of course not! You don’t have to be a lepidopterist to know that big ones will generally have more hairs than little ones.
• Is it an average across all caterpillars? Possibly, but in that case it would be nice to know how variable caterpillars are in hairiness.
• How is the figure of 63,000 arrived at? It seems unlikely that anyone has actually counted that many hairs. Perhaps it is an estimate; perhaps it is more like a guess.

Oak processionary caterpillars

Wikipedia gives the same information in somewhat more careful terms: “The backs of older caterpillars are covered with up to 63,000 pointed defensive bristles”. This statement suggests that the figure of 63,000 is an extreme case rather than some sort of average. This is better, but still not satisfactory. Suppose you knew nothing whatever about the people’s heights and someone told you that people can be up to 8’11” tall. That statement is correct, but it gives you no idea at all of the heights of typical people. In that respect it is very misleading.

A Google search with the question “how do you count the hairs on a caterpillar?” doesn’t turn up anything very promising, but it is likely that someone somewhere has actually done some counting. Perhaps the technique is to count the hairs on a few square millimetres of caterpillar skin, and then measure the total area of hairiness and scale the figure up. Nothing wrong with that, but it is obviously not going to give a very precise answer.

Good statistics are expressed cautiously and with due regard for accuracy, precision and variability. If the story had said “caterpillars of this type can have tens of thousands of hairs” it would have passed harmlessly under the goodstats radar – but an extreme figure of dubious validity quoted as if it is the norm sells papers.

The online version of this article can be read here.

PS The Daily Telegraph website has a headline “Lethal errors in 2 million prescriptions”. If true, that would imply that vast numbers of people are dying from prescription errors – because that is what the word “lethal” means. The story itself refers to “potentially life threatening errors”, but does not reveal a single instance in which a prescription error has killed a patient. Just another example of using an extreme figure of dubious validity to sell a paper.

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Neil Sheldon has taught at The Manchester Grammar School for 40 years. He is a Chartered Statistician and Fellow of the Royal Statistical Society. He has been an RSS Guy Lecturer since 2007.  He is also course leader for the Certificate in Teaching Statistics offered by the RSS Centre for Statistical Education

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