William O’Neil

William O’Neil was an American investor, publisher and researcher, born in Oklahoma City in 1933 and raised in Texas. He studied business at Southern Methodist University, served in the United States Air Force, and began his career as a broker at Hayden, Stone and Company in 1958.

His distinctive move was empirical. Rather than reasoning from theory about what ought to work, he assembled historical data on shares that had produced the largest price gains and looked for characteristics they had shared before the gains occurred. That study became the basis of everything he did afterwards, and it required building historical databases at a point when almost nobody had them in usable form.

He founded William O’Neil and Company in 1963 and bought a seat on the New York Stock Exchange at thirty, and the firm went on to supply computerised historical stock data to institutional clients. In 1984 he launched Investor’s Business Daily as a competitor to the established financial press, built around the data and metrics his research had identified rather than around narrative reporting.

His framework was published as CAN SLIM in How to Make Money in Stocks in 1988. The acronym covers current quarterly earnings, annual earnings growth, new products or management or price highs, supply and demand in the shares outstanding, leader rather than laggard within an industry, institutional sponsorship, and the direction of the overall market. He passed away in 2023.

O’Neil’s method starts from an unusual question. Instead of asking what a good business looks like, he asked what the shares that had actually produced the largest gains looked like before they produced them, and then treated the shared characteristics as a screen. That is a historical and statistical exercise rather than a valuation one, and it produced conclusions that sit awkwardly with classical value investing.

The most awkward is his treatment of price. His data suggested that shares making new highs had gone on to do better than shares making new lows, which inverts the instinct to buy what has fallen. His explanation was that a new high reflects an absence of overhead supply from earlier buyers waiting to break even, and that strength is information rather than a warning.

Earnings acceleration does most of the analytical work in the framework. He weighted recent quarterly earnings growth heavily and looked for it to be accelerating rather than merely present, on the reasoning that the historical winners had almost always shown a sharp improvement in reported results before the largest part of their price move.

The part that keeps the method survivable is the exit rule, and he was more insistent about it than about any of the selection criteria. He advocated cutting a position at a fixed loss from the purchase price, without renegotiating, because the framework accepts that most selections will be wrong and depends on the losses being small when they are.

CAN SLIM gave individual investors a checklist derived from historical data at a time when most retail advice was either narrative or purely valuation based, and the criteria are still built into screening tools that never name him.

Investor’s Business Daily changed what a financial newspaper could look like by leading with relative strength and earnings metrics rather than with reporting, and the metrics it popularised became standard in retail screeners.

His fixed loss-cutting rule is probably his widest influence. The practice of deciding an exit level before buying is now common advice among active investors, and much of that lineage runs through this book.