The cliché “correlation is not causation” has an element of truth to it. Actually, MORE than just an element of truth. Too often we are handed a fait accompli of “scientific proof” that is based on correlative studies, but which has not established a confirmed cause and effect relationship. The following quotes from Ed Tufte (Beautiful Evidence) are relevant to establishing actual requirements:
“Correlation is not causation but it sure is a hint.”
“Empirically observed covariation is a necessary but not sufficient condition for causality.”
“Many true statements are too long to fit on a PP [PowerPoint] slide, but this does not mean we should abbreviate the truth to make words fit. It means we should find a better tool to make presentations.”
“In assessing causal linking lines, the claim that ‘X acts on Y’ should be tested against competing alternatives, such as the paired arrows of interplay. Another check is to reverse the claim: ‘Might it be instead that Y acts on X?’
“Evidence that bears on questions of complexity typically involves multiple forms of discourse.”
“Evidence is evidence, whether words, numbers, images, diagrams, still or moving. It is all information after all.”
Jump the Spark! 
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