Patterns in financial markets | Predicting the unpredictable

Economist.com:
"Stockmarket crashes can be like buses: you wait ages for one to come along, then three arrive at once. That has not stopped people from trying to understand why they happen, of course. But such understanding is handicapped by two things. The first is that markets are complex systems that are hard to analyse. The second is a widespread assumption that, over three centuries of stockmarket trading as we know it, any patterns that do predict the future will already have been identified and “arbitraged” out of existence by people taking advantage of them."