Occam's razor

Last revised by Daniel J Bell on 23 Jun 2022

Occam's razor (also known as lex parsimoniae), an often cited principle of parsimony, economy, or succinctness used in problem-solving. It has also been expressed as the KISS principle or "Keep it simple stupid!".

It states that among competing hypotheses, the one with the fewest assumptions should be selected. Although other, more complicated solutions may ultimately prove correct, in the absence of certainty the fewer assumptions made, the better. By extension, it is often more sensible to consider any patient's clinical presentation as the result of a single condition rather than multiple concurrent diagnoses.

But, it is also important to not overlook Hickam's dictum, i.e. "patients can have as many diseases as they darn well please"; this is especially germane in the modern healthcare setting in which multimorbidity is the rule rather than the exception.

References 3 and 4 below nicely illustrate the tensions between Hickam's dictum and Occam's razor in modern medicine 3,4.

Who does not remember university, where loads of students were (and constantly are) told "When you hear hoofbeats, look for horses, not zebras," which means look for the simplest, most common explanation first.

A more useful approach may be to think of the probability of the diagnosis in general, with decreasing probability down the list:

  1. typical manifestation of a common disorder
  2. atypical manifestation of a common disorder
  3. typical manifestation of a rare disorder
  4. atypical manifestation of a rare disorder

Or conversely:

Often, suggesting rare diseases will rarely prove right.

History and etymology

William of Occam (1285-1348) was a member of the Franciscan order of friars, who hailed from the village of Occam (or Ockham) in the county of Surrey in England. He is remembered as an important philosopher of the medieval period. The fundamental belief that the simplest explanations should be sought for phenomena harks from the Classical Greek period, but William of Occam brought it to the attention of many more through his writings. This was expressed by him in Latin as "entia non sunt multiplicanda praeter necessitate" or "entities must not be multiplied beyond necessity". It is important to note that he was not thinking about the diagnostic method in medicine when he wrote his razor.

The use of the word razor in this context sounds odd to the modern ear, but it is used in philosophy to describe an underlying proposition that permits the 'shaving off' of superfluous explanations!

'Lex parsimoniae' is the Latin for law of parsimony.

See also

ADVERTISEMENT: Supporters see fewer/no ads

Cases and figures

  • Case 1: potassium chloride tablet in trachea
    Drag here to reorder.
  • Updating… Please wait.

     Unable to process the form. Check for errors and try again.

     Thank you for updating your details.