Wednesday, January 23, 2013

Causation and correlation again

A recurring theme in this blog is that correlation is not causation. Yet it's easy, and tempting, to make the mental leap from correlation to causation without sufficient information. Today's example is this column from Lindsay Abrams in The Atlantic. Titled "When Trees Die, People Die" the column describes a study that appears in the American Journal of Preventive Medicine about the relationship between trees and human health.

Here's how Abrams describes the study:
Within four years of first becoming infested, the ash trees die -- over 100 million since the plague began. In some cases, their death has an immediate impact, as they fall on cars, houses, and people. In the long term, their disappearance means parks and neighborhoods, once tree-lined, are now bare.
Something else, less readily apparent, may have happened as well. When the U.S. Forest Service looked at mortality rates in counties affected by the emerald ash borer, they found increased mortality rates. Specifically, more people were dying of cardiovascular and lower respiratory tract illness -- the first and third most common causes of death in the U.S. As the infestation took over in each of these places, the connection to poor health strengthened.
Note the use of the word "may" in the first sentence of the second paragraph, and the conclusion in the last sentence. But the post overstates the conclusions of the study, which is available here. The authors, having surveyed the literature, conclude that their study strengthens the correlation, but reiterate that they can conclude nothing about causation.

It's the kind of thought-provoking study that should inspire more studies. Let's hope it does.

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