William Sharpe himself conceded that Roll's critique of the tests of his model was essentially correct. Sharpe told Institutional Investor's reporter: "We cannot, without a shadow of doubt, establish the validity of the capital asset pricing model." (Wallace 1980, p. 24) Stambaugh's study meant that the Capital Asset Pricing Model is "maybe OK after all," says Sharpe (interview). Nevertheless, in his view that kind of empirical defense against criticism of the model misses two deeper points. First, "nobody can confirm a hypothesis" such as the CAPM. "You can only fail to disconfirm it." (Sharpe interview) Second, statistical difficulties (such as the large sampling errors in estimating the average return on a stock), economic and historical change, and variation across countries render empirical findings in this area unstable.

Donald MacKenzie, An Engine, Not a Camera, loc. 408