Kouska's fallacy: the error of the divided denominator

The significance of two normally independent findings in a single patient is often difficult to assess. One simple measure of the likelihood of random coincidence is the joint probability, based on the assumption that the findings are independent. Unfortunately, this technique is often used wrongly. The error is the division of an event into its components, followed by calculation of their joint probability. These two processes, used together, make the likelihood of a common event happening seem very improbable. This can be termed the error of the divided denominator, or Kouska's fallacy, after a fictional character who used this technique to disprove the existence of life.