This article was written by Joseph Rickert.
We all "know" that correlation does not imply causation, that unmeasured and unknown factors can confound a seemingly obvious inference. But, who has not been tempted by the seductive quality of strong correlations?
Fortunately, it is also well known that a well done randomized experiment can account for the unknown confounders and permit valid causal inferences. But what can you do when it is impractical, impossible or unethical to conduct a randomized experiment? (For example, we wouldn't want to ask a randomly assigned cohort of people to go through life with less education to prove that education matters.) One way of coping with confounders when randomization is infeasible is to introduce what Economists call instrumental variables. This is a devilishly clever and apparently fragile notion that takes some effort to wrap one's head around.
On Tuesday October 20th, we at the Bay Area useR Group (BARUG) had the good fortune to have Hyunseung Kang describe the work that he and his colleagues at the Wharton School have been doing to extend the usefulness of instrumental variables. Hyunseung's talk started with elementary notions: like explaining the effectiveness of randomized experiments, described the essential notion of instrumental variables and developed the background necessary for understanding the new results in this area. The slides from Hyunseung's talk available for download in two parts from the BARUG website. As with most presentations, these slides are little more than the mute residue of talk itself. Nevertheless, Hyunseung makes such imaginative used of animation and build slides that the deck is worth working through.
The following slide from Hyunseung's presentation captures the essence of the instrumental approach.
To read more about Instrumental Variables, click here.
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Comment
Unfortunately it's not easy to find these instrumental variables. And if/when you do, they bring with them their own host of concerns.
See also:
Nontransitivity, Correlation, and Causation. Langford, E., Schwertman, N., and Owens, M. (2001), “Is the Property of Being Positively Correlated Transitive?” The American Statistician, 55, 322-325: Comment by Friedman.
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