The rate of attempted rapes of girls and women in the United States decreased 46% from 1973 to 1987, according to a study released by the Justice Department. The rate of attempted rapes fell from 1.3 per 1,000 girls and women in 1973 to 0.7 per 1,000 in 1987, the study said. However, the rate of completed rapes--0.6 per 1,000--held steady over the same time span. Only 53% of rapes or attempted rapes are reported to police, the study said. The report was compiled by extrapolating figures from a survey of 49,000 households.