NEW YORK (Bloomberg) — Ace Ltd., the Bermuda-based insurer that's moving its legal home to Switzerland, had its stock recommendation raised to "buy" by Citigroup Inc. analyst Joshua Shanker after a 15-percent, six-day drop in the shares.

"We view the recent dip as an attractive buying opportunity," Shanker said in a research note published after the close of trading yesterday.

The share drop compares with a 6.6 percent decline in the 24-stock KBW Insurance Index over the same time.

Ace advanced $1.97, or 4.2 percent, to $49 in New York Stock Exchange composite trading yesterday.

Ace were replaced on the S&P 500 Index by MasterCard Inc. after the close of trading yesterday, and Mr. Shanker said investors may have been selling the shares because of its exit from the basket of stocks.