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Tuesday, June 07, 2005

Lancet: Extra Credit Assignment


I've occasionally debated the Lancet study with their blogging defenders for some months now. It's been frustrating, and I've finally figured out why. The Lancet defenders, such as Lambert and Davies, both know something about the kind of statistics that starts "assume you have a random sample," but aren't particularly knowledgeable about the actual mechanics of obtaining a random sample.

The most egregious example is Lambert's casual dismissal of the cogent and well-informed criticisms of Professor Stephen Apfelroth of the Albert Einstein Medical school. Lambert dismisses Apfelroth's criticisms as "just speculations the sampling was not done correctly." In fact, Apfelroth's criticisms are hardly speculations: they're the expert opinion of somebody who obviously understands survey methods. They could be taken straight from a textbook on sampling. Here, Lambert is just sneering from ignorance, and I doubt that he understands Apfelroth's criticisms, which are indeed pretty terse.

One of Apfelroth's more obscure, but more important criticisms is this:
When a town or village was selected from the "cumulative population lists for the Governorate", the survey team then "drove to the edges of the area and stored the site coordinates"....it seems quite likely that the grid rectangles created by driving around in a war zone were much smaller than the original census tracts used in the "cumulative population lists".
If you understand survey methods, you'll understand why it is a huge problem that the grid rectangles were much smaller than the original census tracts. And if you think about this issue further, you'll realize that the Lancet method is highly likely to oversample rural areas and the fringes of cities. It is precisely these low-density areas where the fighting was most intense.

So here's the extra credit question. If you get it right, I promise to take your criticism or praise of the Lancet study very seriously. Why does the discrepancy between the grid rectangles and the census tracts imply that the Lancet study oversamples low-density areas?

UPDATE: So, as of June 10, no one's attempted my quiz, even though I know plenty of people have come over here from the Lancet discussion on Tim Lambert's blog. Come on people! I'll settle for an explanation of what Apfelroth is talking about!
 
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