When the Vanguard Group of Malvern, Pa. began offering its Admiral shares in November, it did so claiming it wanted to reward its largest, most loyal investors. But the product is more likely designed to prevent shareholders from straying to exchange-traded funds, according to industry observers.

The new shares, which offer lower expense ratios for investors with accounts of significant size and tenure, have proven to be very popular, said D. Christopher Brown, vice president of research with Financial Research Corporation of Boston. Admiral shares attracted $9.6 billion in November and an additional $2.7 billion in December, he said.

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