Sunday, May 22, 2011

Selection effects, Buffett's rebuttal, and the causality question

Some thoughts on causality based on a story I recall from Alice Schroeder's  The Snowball, Warren Buffett's biography. (I read the book over two years ago, and it was a library copy, so I can't be sure of the details, but I'm sure of the logic.)

Warren Buffett attended a conference on money management where he made a big splash against a group of efficient market advocates. Efficient financial markets imply that, in the long term, it's impossible to have returns above market average, something that Buffett had been doing for several years by then.

The efficient markets hypothesis advocates present at this conference made the predictable argument against reading too much in the outsized returns of a few money managers: if there's a lot of people trading securities, then some will do better than the median, while others will do worse than the median, just as an artifact of the randomness. To over-interpret this is to imagine clusters where none exists.

Buffett then told a parable along the following lines: "Imagine that you look at all the money managers in the market last year, say 20,000, and see that there are 24 that did much better than the rest of the 20,000. So far it could be the case of a random cluster, yes. Then you find those 24 traders, and discover that 23 came from a very small town, [Buffett gave it the name of a mentor, but I can't recall it] Buffettville. Now, most people would think that there's something in Buffettville that makes for good managers; but you are telling us that it's all a coincidence."

Buffett's argument carries some weight in the sense that the second variable (i.e. being from Buffettville) is not a-priori related to having higher returns, so it must be related by a hitherto unknown causality relationship.

But there's a problem here. Even if a large proportion of the successful managers are from Buffetville, that doesn't mean that being from Buffettville makes people better managers; it might be the case that there were many other Buffettville managers in the 20,000 and those were at the very bottom. That would mean that managers from Bufettville have a much higher variance in returns than the market, and that the results, once again were the result of randomness.

My argument here is that the story as I recall it being told in Schroeder's book is an incomplete rebuttal of the efficient markets hypothesis, not a defense of that hypothesis. I'm not a finance theorist; I'm in marketing, where we do believe that some marketers are much better than others, so I have no bone to pick either with the theory or its critics.

I'm just a big fan of clear thinking in matters managerial or business.