| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.0275 |
| 2021 | 6.10008 |
| 2020 | 6.25657 |
| 2019 | 6.41314 |
| 2018 | 6.53317 |
| 2017 | 6.6168 |
| 2016 | 6.63999 |
| 2015 | 6.63455 |
| 2014 | 6.67993 |
| 2013 | 6.7961 |
| 2012 | 6.93994 |
| 2011 | 7.1045 |
| 2010 | 7.27713 |
| 2009 | 7.40092 |
| 2008 | 7.47634 |
| 2007 | 7.54082 |
| 2006 | 7.58142 |
| 2005 | 7.57699 |
| 2004 | 7.54688 |
| 2003 | 7.49443 |
| 2002 | 7.38566 |
| 2001 | 7.3816 |
| 2000 | 7.28971 |
| 1999 | 6.98371 |
| 1998 | 6.76031 |
| 1997 | 6.66445 |
| 1996 | 6.47778 |
| 1995 | 6.26651 |
| 1994 | 6.16644 |
| 1993 | 6.05566 |
| 1992 | 5.88925 |
| 1991 | 5.59695 |
| 1990 | 5.42268 |
| 1989 | 5.40172 |
| 1988 | 5.29025 |
| 1987 | 5.12845 |
| 1986 | 5.09258 |
| 1985 | 5.06821 |
| 1984 | 4.96984 |
| 1983 | 4.96973 |
| 1982 | 5.03338 |
| 1981 | 5.11012 |
| 1980 | 5.29014 |
| 1979 | 5.62399 |
| 1978 | 5.83913 |
| 1977 | 5.91693 |
| 1976 | 5.9932 |
| 1975 | 5.93427 |
| 1974 | 5.71759 |
| 1973 | 5.68039 |
| 1972 | 5.76828 |
| 1971 | 5.72837 |
| 1970 | 5.79316 |
| 1969 | 5.99321 |
| 1968 | 5.94756 |
| 1967 | 5.75173 |
| 1966 | 5.74366 |
| 1965 | 5.73696 |
| 1964 | 5.65474 |
| 1963 | 5.65681 |
| 1962 | 5.78843 |
| 1961 | 5.91852 |
| 1960 | 6.01726 |