| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.55585 |
| 2021 | 6.54805 |
| 2020 | 6.55069 |
| 2019 | 6.54997 |
| 2018 | 6.53036 |
| 2017 | 6.4841 |
| 2016 | 6.40894 |
| 2015 | 6.31679 |
| 2014 | 6.24506 |
| 2013 | 6.21368 |
| 2012 | 6.23211 |
| 2011 | 6.33842 |
| 2010 | 6.52394 |
| 2009 | 6.71119 |
| 2008 | 6.8611 |
| 2007 | 6.96723 |
| 2006 | 7.01907 |
| 2005 | 7.05947 |
| 2004 | 7.16799 |
| 2003 | 7.3499 |
| 2002 | 7.54992 |
| 2001 | 7.75741 |
| 2000 | 7.95566 |
| 1999 | 8.09918 |
| 1998 | 8.19998 |
| 1997 | 8.28086 |
| 1996 | 8.32549 |
| 1995 | 8.32762 |
| 1994 | 8.30399 |
| 1993 | 8.2533 |
| 1992 | 8.15106 |
| 1991 | 8.14343 |
| 1990 | 8.05235 |
| 1989 | 7.7344 |
| 1988 | 7.51161 |
| 1987 | 7.47628 |
| 1986 | 7.33085 |
| 1985 | 7.1 |
| 1984 | 7.02212 |
| 1983 | 6.9147 |
| 1982 | 6.65035 |
| 1981 | 6.29843 |
| 1980 | 6.09013 |
| 1979 | 5.99508 |
| 1978 | 5.82689 |
| 1977 | 5.62726 |
| 1976 | 5.52752 |
| 1975 | 5.47108 |
| 1974 | 5.39659 |
| 1973 | 5.36195 |
| 1972 | 5.36942 |
| 1971 | 5.39038 |
| 1970 | 5.42994 |
| 1969 | 5.56763 |
| 1968 | 5.70276 |
| 1967 | 5.80913 |
| 1966 | 5.99515 |
| 1965 | 6.18031 |
| 1964 | 6.26413 |
| 1963 | 6.51815 |
| 1962 | 6.81137 |
| 1961 | 6.8595 |
| 1960 | 6.96128 |