Stocks will be "partially weighted," or shared, across different size indexes -- so, as a company grows or shrinks, it doesn't have to be added or eliminated in one fell swoop. Any additions or deletions will be made in "packets," or gradual steps over time, and the days on which the substitutions take effect will be randomized.

These techniques should reduce turnover and poaching by outsiders while preserving the "style purity" of the indexes, argues Pastor.

New screening methods could also enable CRSP to offer investors an index of stocks that rank high in both growth and value.

"That's a very interesting idea that I've wanted to be able to index for a long time," says Gus Sauter, the chief investment officer at Vanguard Group.

David Barclay, CRSP's chief operating officer, says "a couple of" index-fund providers have expressed interest in the new benchmarks. Sauter says that "it's something we'll have to consider." CRSP's approach, he adds, is "well thought-out" and "compelling."

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There is other good news for index investors. Over the past year, Charles Schwab (SCHW, news), TD Ameritrade (AMTD, news) and Vanguard have eliminated brokerage commissions on selected exchange-traded funds.

William Koehler, the chief investment officer at ETF Portfolio Partners in Leawood, Kan., says he has opened custodial Roth IRAs for both of his teenage sons at Schwab, where the minimum opening balance for such accounts is $100.

"They mow lawns, and now I can buy ETFs for them with their lawn-mowing money without paying a commission," he says. "When they're in their 20s, they'll thank me."

This article was reported by Jason Zweig for The Wall Street Journal.