Sunday, April 6, 2008

3rd variable illustration


'Healthier hearts' for cat owners

However the authors warned against impulsive cat purchases.

They said while cats may indeed have a calming effect, it was unclear whether the kind of people who opted for a cat in the first place may have a lower risk of heart attack.

read the BBC story

The quote directly addresses the third variable problem: could something else (such as a personality trait) cause both a lower risk of heart disease and cat ownership? But also, there's the directionality problem: could people who are less likely to develop heart disease prefer cats? If you think about this for a second, you realize that this explanation is either silly or the 3rd variable problem in disguise.

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