In a 1929, pre-crash speech, esteemed economist Irving Fisher became notorious for declaring that U.S. stock prices had reached “what looks like a permanently high plateau,” writes Jason Zweig of The Wall Street Journal.

Citing data from then-renowned statistician Karl Karsten, Fisher argued that the stock market was “up to 25% overvalued by early 1929.” In 1931, however, Karsten wrote a book outlining the flaws in his prediction method and warned against forecasting in general, the article says. And after a subsequent, deeper analysis of his data, Zweig writes, Karsten “made discoveries that anticipated many of the ideas behind hedge funds and so-called smart beta, or mechanical strategies to earn excess returns, that are so popular today.”

At the end of 1930, Karsten launched a small fund run with “real money and nothing but math.” While the fund performed well at first, Zweig writes, Karsten warned that such performance was not likely to continue if his strategy was used by too many investors. The article reports that Karsten’s fund seems to have disappeared by 1942.

Zweig concludes that, if Karsten were alive today, he would warn investors against using past data to predict market activity going forward. “When someone uses historical data to forecast a cataclysmic market crash with near-certainty, bear in mind that the patterns of the past may no longer hold. And remember that strategies like ‘smart-beta’ are likely to work best when most investors doubt they will work at all.”