By Meg Richards
The Associated Press

NEW YORK - Wall Street professionals know to keep their expectations in check in September, historically the worst month for stocks. Money managers are getting back to business, cleaning house, and often sending the market lower in the process.

September has opened strong eight of the last nine years, but it has ended with a knock-out punch for stocks for the last five, partly because institutional traders are making end-of-the quarter portfolio changes. But some analysts, noting the market's unusual sell-off this July, question whether the pattern will hold true this year.

"The normal election-year pattern shows the market rallying in July and most of August, then selling off, so we've been much weaker than normal," said Tim Hayes, global stock strategist at Ned Davis Research in Venice, Fla. "The market may have already had that pre-election sell-off."

During seven of the last 10 presidential races, the major indexes posted gains for September, according to the Stock Trader's Almanac. The month ended in a loss three times - in 1972 and 1984, when incumbents ran and won, and in 2000, when there was no incumbent.

Part of what drives September's typical weakness is the difficulty of assessing the outlook for third-quarter earnings after the summer doldrums. It's almost as if earnings for the entire 12-week period depend on this month, says Tobias Levkovich, chief U.S. equity strategist at Citigroup's Smith Barney division. And when Labor Day falls deeper in the month, as it does this year, things can get even more complicated.

"We suspect that there will be a fair amount of nervousness about earnings this quarter, especially if the August employment numbers ... are less than exciting," Levkovich told investors.

The Labor Department's data report released Friday offered some promise after two months of anemic jobs growth, but it wasn't enough to electrify the market. Employers added 144,000 new jobs last month, just short of the 150,000 economists were looking for.

History offers some guidance about which areas are most vulnerable to seasonal weaknesses. On average, Levkovich found that telecommunications, utilities and healthcare have posted gains, while consumer staples, materials, industrials, information technology and consumer discretionaries have shown weaker performances.