"It's not something that can be run on autopilot, despite the use of computer models," said Greg Carlson, an analyst at Morningstar, the mutual fund tracker. "You have to have a research team that's going to continue to look for new factors to add to the models because oftentimes the efficacy of a particular factor is either not recognized by the models or gets arbitraged away over time."

Mr. Mortimer of Charles Schwab said human managers at the company halted purchases of Merck in 2004 during the Vioxx controversy even though their models were calling for purchases of the stock. Merck's shares declined roughly 25 percent from the beginning of the problems through the second quarter this year.

There is another possible drawback of quantitative funds: because of the high volume of buy and sell signals generated by the models, the funds have higher-than-average turnover ratios and are therefore not as tax-efficient as index funds. This same factor also leads fund companies to limit the size of the funds.

Even though quant funds have performed well in recent years, Joel M. Dickson, head of Vanguard's active quantitative equity group, said investors should be careful about jumping into such funds now.

"I'm a little concerned that investors may be chasing performance," Mr. Dickson said. The most recent three years have been very good for many quant funds, he said, but he cautioned that their fundamental strategy may become less effective: as their stocks become more accurately priced, it becomes harder for quant managers to beat their benchmarks.

One of the best-performing funds so far in 2006 is the Bridgeway Ultra-Small Company fund, which uses five quantitative models to fill out its holdings with an assortment of growth, value and momentum stocks. The fund, now closed to new investment, gained 18 percent this year through June.

Still, it's rare to see a quantitative fund at the top of the performance charts, since many funds of this type aim only to outperform a benchmark.