| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.54493 |
| 2021 | 6.67898 |
| 2020 | 6.77681 |
| 2019 | 6.82481 |
| 2018 | 6.82953 |
| 2017 | 6.80373 |
| 2016 | 6.77427 |
| 2015 | 6.78269 |
| 2014 | 6.81554 |
| 2013 | 6.84646 |
| 2012 | 6.88509 |
| 2011 | 6.93315 |
| 2010 | 6.98124 |
| 2009 | 7.04031 |
| 2008 | 7.1257 |
| 2007 | 7.24031 |
| 2006 | 7.37434 |
| 2005 | 7.50859 |
| 2004 | 7.61969 |
| 2003 | 7.67733 |
| 2002 | 7.67424 |
| 2001 | 7.63683 |
| 2000 | 7.58768 |
| 1999 | 7.52937 |
| 1998 | 7.48128 |
| 1997 | 7.45845 |
| 1996 | 7.45708 |
| 1995 | 7.46995 |
| 1994 | 7.48931 |
| 1993 | 7.48546 |
| 1992 | 7.43626 |
| 1991 | 7.34948 |
| 1990 | 7.24618 |
| 1989 | 7.16599 |
| 1988 | 7.12734 |
| 1987 | 7.14722 |
| 1986 | 7.28165 |
| 1985 | 7.52703 |
| 1984 | 7.7732 |
| 1983 | 7.98941 |
| 1982 | 8.18386 |
| 1981 | 8.30816 |
| 1980 | 8.408 |
| 1979 | 8.60113 |
| 1978 | 8.903 |
| 1977 | 9.25619 |
| 1976 | 9.61993 |
| 1975 | 9.95143 |
| 1974 | 10.1869 |
| 1973 | 10.335 |
| 1972 | 10.4624 |
| 1971 | 10.559 |
| 1970 | 10.603 |
| 1969 | 10.6051 |
| 1968 | 10.5538 |
| 1967 | 10.4126 |
| 1966 | 10.424 |
| 1965 | 10.2964 |
| 1964 | 9.88747 |
| 1963 | 9.65736 |
| 1962 | 9.63536 |
| 1961 | 9.48707 |
| 1960 | 9.30774 |