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Views And Reviews

Impossible errors

BMJ 2018; 361 doi: (Published 05 June 2018) Cite this as: BMJ 2018;361:k2427
  1. Giles Maskell, radiologist
  1. Truro, UK
  1. gilesmaskell{at}

Hindsight bias is a real and very powerful phenomenon, and not just in radiology

There is a discipline in academic law concerned with “impossible crimes.” These are “crimes which are not crimes.” One classic example involves an international traveller who is found to be in possession of a bag of white powder which he believes to be cocaine, but which on testing turns out to be a bland substitute, say chalk dust. Importing chalk dust is not a crime so on the face of it no crime has been committed. It’s an impossible crime.

I would like to adopt a similar formulation to describe events which sometimes happen in radiology and could be designated “impossible errors,” that is to say errors which are not errors. Consider Ms X who undergoes a computed tomography (CT) examination because of bowel symptoms. The scan reveals a tumour in …

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