Certainly one should not underestimate the complexity of judging whether patterns of market prices, for example, have moved toward greater conformity with a model such as Black-Scholes-Merton. One way of formulating the question is to examine the extent to which the model's predictions are borne out. However, what a model predicts is often not straightforward. The BlackScholes-Merton model, for example, yields an option price only after the characteristics of the option and the values of the parameters of the Black-Scholes equation have been set. One parameter, the volatility of the stock, is acknowledged not to be directly observable, so there is no unique theoretical price to compare with "actual" prices.

Donald MacKenzie, An Engine, Not a Camera, loc. 102