Conversely, central to the Capital Asset Pricing Model was what the model posited about the returns expected by investors on assets with different sensitivities to market fluctuations, but typically no attempt was made to measure these expected returns directly for example, by surveying investors. (The results of any such survey would have been regarded as unreliable by most financial economists.) Instead, in empirical tests of the Capital Asset Pricing Model, more easily measurable after-the-fact realized returns were used as a proxy for expected returnsa substitution that rested on an efficient-market, rational-expectations view of the latter.

Donald MacKenzie, An Engine, Not a Camera, loc. 111