Friday, May 20, 2011


The post-hoc or more formally the post-hoc ergo propter hoc, is a logical fallacy where one event precedes another event and is thus found to be the cause of the second event. The basic format can be expressed as:

P1. A came before B,
C. therefore A caused B.

A silly example would be "Roosters crow before the sun rises, therefore roosters crowing cause the sun to rise". A more common example would be "Bob installed some new software on his computer. The next time he turned on the computer it crashed. Bob concluded that the new software caused the computer to crash". Now it is entirely possible that the new software did cause the computer to crash, but it is also possible that a computer virus or possibly a power spike caused it to crash. The point is that a positive correlation alone is generally not sufficient evidence to prove causation. Post-hoc arguments are often even shakier than an incorrect inference from correlation to causation in that there is typically only one anecdote or event in which the inference is being made from.

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