Leading economic indexes in seven of the world's nine major industrialized countries are strengthening, a signal that growth forecasts may have to be revised upward, the Conference Board reported. The research organization said the index for the United States, which had shown almost no growth three months ago, increased at an annual rate of 4% in December. The strongest rate was Australia's at 13%, the same as in September, followed by Taiwan at 10%, compared to 6% three months ago. Italy was at 9%, up from 8%; West Germany at 8%, up from 7%, and Canada at 7%, up from 5%. France's rate was 5%, a 1 percentage point drop. The major exceptions were Japan, where the annual rate weakened to 1% from 9%, and Britain, which dropped to minus 1% from 2%, the board said.