Conditioning on a Collider, Human Popsicle Edition

July 30, 2011 at 2:17 am 1 comment

| Gabriel |

Robin Hanson mentions a poll of whether people would like to be cryogenically frozen and notes that the rate is higher for people outside the US, who he assumes are American expats and uses that as an argument about adventurous personality types. I’m skeptical that most of these people are American citizens as compared to foreigners who speak English, but let’s put that aside. The big problem is that this is a self-selected reader poll.

The short version of the problem is that reader polls are nearly worthless. The long version is that you’d expect to find a negative correlation between access to the poll and salience of its subject matter. Assume that there are two things driving participation in a reader poll:

  1. Accessibility of the poll. In this case, the poll was hosted by ABC News, an American news organization that is presumably of greatest interest to people living in the US.
  2. Salience of the poll’s subject matter. It’s pretty easy to imagine that cryonics fans may seek out material about cryonics. Some of them might have Google News alerts. Likewise it’s not exactly unheard of for fans of a band/politician/whatever to find out about a reader poll and direct other fans to it. Note that I’m assuming that for this issue salience is highly correlated with favorability.

If so, the magic of conditioning on a collider means that the subset of the population that responds to the reader poll will have an artifactual negative correlation between accessibility and salience. Anytime that censorship is related to the interaction of two variables then the observed data suffer artifactual results about the relationship between those two variables (and their close correlates).

That’s it. We’re done. No need to speculate about Indiana Jones and the Freezer Burn of Doom.


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1 Comment

  • 1. Brian  |  August 2, 2011 at 3:00 am

    I saw this poll. It was right under the poll that predicts Alf Landon beating Obama in a landslide in 2012…

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