Steve Chapman

Momentum also counts for nothing in the stock market. What happened this week with stock prices does not foretell what will happen next week. Economists generally agree that in a well-functioning capital system, the market is unpredictable.

As they put it, changes in stock prices follow the pattern of a "random walk" -- which is to say no pattern at all. If a stock price goes up today, you can bet it will also go up tomorrow, unless it goes down. Looking backward to guess where it is headed is like driving with your eyes on the rearview mirror.

Sports are particularly susceptible to this alluring metaphor. But it's grossly unreliable. This year, the Detroit Tigers and St. Louis Cardinals collapsed at the end and barely made the postseason -- giving them, by my count, zero momentum. Each then won the first round of the playoffs.

The Wall Street Journal noted the other day that in the last five postseason series they lost, the New York Yankees took the early lead in each. Where was momentum then? As the former Kansas City and Cincinnati manager Bob Boone once said, "I don't believe in momentum. Momentum changes with one hanging curveball."

When a team wins the first two games of the World Series, true, it usually goes on to win the whole thing. But that's not because of momentum -- that's because it has a two-game lead. If one sprinter begins a 100-yard dash 50 yards from the finish line, she'll probably beat the other runners, even though she has no more momentum at the start.

Looked at carefully, this alleged invisible force is just an empty metaphor that explains nothing. But at this point, it's probably impossible to stamp out. Because you know what it's got on its side.

Steve Chapman

Steve Chapman is a columnist and editorial writer for the Chicago Tribune.

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