Quote from weewilly:
btw did you ever hear the joke about the efficient market theory economist walking down the street with his wife? she sees a dollar on the sidewalk and says "look, there's a dollar on the sidewalk." the economist says "no, there can't be a dollar on the sidewalk because someone would have picked it up already."
so much for theories....
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wee
As long as you brought up efficient market theory...
I did my graduate work at U of Chicago and got a good dose of it. I even went to the library and read Fama's doctoral dissertation where he laid it out.
News flash: the efficient market hypothesis only refers to the expectation of price change, namely that it is essentially zero. There is never a claim with respect to variance (or its square root standard deviation which plays the role of volatility in the BS formula). Simply put, efficient market theory (at least in its original formulation) never says that the market is efficient with respect to volatility.
One of Fama's advisors was Benoit Mandelbrot who invented fractal geometry. Mandelbrot showed that changes in stock prices were distributed according to a symmetric distribution...thus Fama's claim that E(price change) = 0. But Mandelbrot also showed that the distribution is typically not Normal and in fact that the variance is not defined for most price change distributions. When you try to compute it the equations blow up.
Three important implications: 1 ) price changes can be discontinuous; 2) distributions will have fat tails and 3) BS results are only approximations. All of this was pre-1987. Mandelbrot correctly predicted the failure of so-called portfolio insurance.
So what, you might say. Well, there is room in efficient market theory for systematically making money from options. So the economist in your joke should have said "ah, a short straddle" and pocketed the buck...all the while making sure he wasn't about to get run over by a truck.