<P> In statistics, many statistical tests calculate correlations between variables and when two variables are found to be correlated, it is tempting to assume that this shows that one variable causes the other . That "correlation proves causation," is considered a questionable cause logical fallacy when two events occurring together are taken to have established a cause - and - effect relationship . This fallacy is also known as cum hoc ergo propter hoc, Latin for "with this, therefore because of this," and "false cause ." A similar fallacy, that an event that followed another was necessarily a consequence of the first event, is the post hoc ergo propter hoc (Latin for "after this, therefore because of this .") fallacy . </P> <P> For example, in a widely studied case, numerous epidemiological studies showed that women 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 randomized controlled trials showed that HRT caused a small but statistically significant increase in risk of CHD . Re-analysis of the data from the epidemiological studies showed that women undertaking HRT were more likely to be from higher socio - economic groups (ABC1), with better - than - average diet and exercise regimens . The use of HRT and decreased incidence of coronary heart disease were coincident effects of a common cause (i.e. the benefits associated with a higher socioeconomic status), rather than a direct cause and effect, as had been supposed . </P>

The phrase correlation does not mean causation means
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