Fischer Black

Fischer Black appears as a reference point in the book’s examination of how accurately markets can judge the future. His striking standard for efficiency—allowing a stock price to fall between 50 percent and 200 percent of a company’s true value—makes the uncertainty of long-range valuation part of the test rather than treating any large error as proof of failure.

A Demanding Forecast

The standard arises from the difficulty of forecasting a company’s prospects over fifteen or twenty years. Such an estimate must account for which drugs a pharmaceutical company and its competitors may invent, changes in regulation and insurance, shifts in lifestyles and attitudes, the global economy, management continuity, the strength of the company’s research pipeline, scientists’ career choices, and the amount invested in research and development. Those judgments must then be translated into financial projections. Looking back two decades later and finding the resulting estimate accurate would, the book observes, seem miraculous.

Against that background, Black proposed that a market could still be considered efficient if a company’s stock price remained between half and twice its true value. The example makes the margin concrete: for a company truly worth $10 billion, a market value from $5 billion to $20 billion would meet his standard. The range initially sounds startlingly permissive, since it allows an error as large as 100 percent; the uncertainty of predicting two decades of business outcomes is the reason the book asks readers to reconsider what accuracy should mean.

Measuring the Crowd Against Alternatives

Black’s benchmark supports a comparative account of market accuracy, not a claim that markets always give the right answer. The book’s central question is how inaccurate a forecast is compared with available alternatives: being wrong by 100 percent is a serious miss, but it is better than being wrong by 300 percent. The proposed strength of collective judgment is that a group can, on average, produce a better answer consistently than any individual could.

The evidence offered for that claim is that only a tiny fraction of investors consistently outperform the market. The comparison is especially demanding because most investors assess only a small number of stocks, while the market must set prices for more than five thousand. That the market performs better than almost all investors under those conditions is presented as significant evidence for its efficiency, even though the standard does not make every price correct.

The Conditions Behind Collective Judgment

The broader account of market intelligence identifies four conditions that can make an aggregated judgment effective: diversity of opinion, independence, decentralization, and aggregation. People need private information or differing interpretations; their views must not simply be determined by those around them; they must be able to draw on specialized or local knowledge; and some mechanism must combine separate judgments into a collective result. When a large enough group is diverse and independent, individual errors can cancel in the average, leaving more of the information people possess.

That mechanism is not a guarantee of wisdom. If the information held by the group is poor, canceling errors cannot produce a sound answer. The book’s argument is that collective verdicts can nonetheless contain a remarkably full picture of the available information, and that their value is best understood through comparison with individual judgment and the difficulty of the problem being predicted.

A Standard, Not a Promise

The twofold range associated with Black gives the efficiency debate a practical scale while exposing its central tension: the longer and more uncertain the forecast, the harder it is to define a correct price. A market may miss the eventual value by a wide margin and still outperform the individual alternatives; that comparative success is not the same as perfect foresight.

The book also stresses that financial markets do not aggregate collective judgment as cleanly as some other forecasting settings. Stock prices can move much more sharply than changes in companies’ underlying values would justify, in part because a company’s distant future is harder to predict than a football result or an election outcome with a definite resolution. Black’s standard therefore functions as a cautious benchmark for assessing market performance, not as evidence that every market movement is rational or every valuation reliable.