| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | - |
| 2021 | 0.475066 |
| 2020 | 0.475811 |
| 2019 | 0.480422 |
| 2018 | 0.482614 |
| 2017 | 0.485162 |
| 2016 | 0.486551 |
| 2015 | 0.488388 |
| 2014 | 0.490283 |
| 2013 | 0.492164 |
| 2012 | 0.493847 |
| 2011 | 0.50183 |
| 2010 | 0.50987 |
| 2009 | 0.518532 |
| 2008 | 0.527552 |
| 2007 | 0.537063 |
| 2006 | 0.54972 |
| 2005 | 0.562644 |
| 2004 | 0.575522 |
| 2003 | 0.58851 |
| 2002 | 0.601397 |
| 2001 | 0.615506 |
| 2000 | 0.621514 |
| 1999 | 0.628469 |
| 1998 | 0.640854 |
| 1997 | 0.651339 |
| 1996 | 0.664477 |
| 1995 | 0.682892 |
| 1994 | 0.691452 |
| 1993 | 0.703096 |
| 1992 | 0.717622 |
| 1991 | 0.733952 |
| 1990 | 0.743826 |
| 1989 | 0.752479 |
| 1988 | 0.759684 |
| 1987 | 0.766613 |
| 1986 | 0.781921 |
| 1985 | 0.789181 |
| 1984 | 0.796205 |
| 1983 | 0.803128 |
| 1982 | 0.810506 |
| 1981 | 0.822584 |
| 1980 | 0.830696 |
| 1979 | 0.838706 |
| 1978 | 0.848013 |
| 1977 | 0.847043 |
| 1976 | 0.855239 |
| 1975 | 0.863404 |
| 1974 | 0.871959 |
| 1973 | 0.88269 |
| 1972 | 0.893514 |
| 1971 | 0.905996 |
| 1970 | 0.920425 |
| 1969 | 0.933722 |
| 1968 | 0.901817 |
| 1967 | 0.87809 |
| 1966 | 0.8939 |
| 1965 | 0.910948 |
| 1964 | 0.927442 |
| 1963 | 0.948912 |
| 1962 | 0.949378 |
| 1961 | 0.983336 |
| 1960 | - |