Correlation Implies Causation?
Posted by pwl on March 8, 2009
Correlation does not imply causation is a phrase used in the sciences and the statistics to emphasize that correlation between two variables does not imply that one causes the other. Its negation, correlation proves causation, is a logical fallacy by which two events that occur together are claimed to have a cause-and-effect relationship. The fallacy is also known as cum hoc ergo propter hoc (Latin for “with this, therefore because of this”) and false cause. By contrast, the fallacy post hoc ergo propter hoc requires that one event occurs before the other and so may be considered a type of cum hoc.
In a widely-studied example, numerous epidemiological studies showed that women who were taking combined hormone replacement therapy (HRT) also had a lower-than-average incidence of coronary heart disease (CHD), leading doctors to propose that HRT was protective against CHD. But controlled trials showed that HRT caused a small and significant increase in risk of CHD. Re-analysis of the data showed that women undertaking HRT were more likely to be from socio-economic groups ABC1, with better than average diet and exercise regimes. The two were coincident effects of a common cause, rather than cause and effect as had been supposed.