| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.36526 |
| 2021 | 6.33977 |
| 2020 | 6.3397 |
| 2019 | 6.35015 |
| 2018 | 6.35303 |
| 2017 | 6.36003 |
| 2016 | 6.37438 |
| 2015 | 6.38999 |
| 2014 | 6.42497 |
| 2013 | 6.49192 |
| 2012 | 6.58679 |
| 2011 | 6.71076 |
| 2010 | 6.84203 |
| 2009 | 6.94382 |
| 2008 | 6.99726 |
| 2007 | 7.00382 |
| 2006 | 6.979 |
| 2005 | 6.9384 |
| 2004 | 6.88801 |
| 2003 | 6.84505 |
| 2002 | 6.81593 |
| 2001 | 6.81158 |
| 2000 | 6.83403 |
| 1999 | 6.85748 |
| 1998 | 6.85602 |
| 1997 | 6.81359 |
| 1996 | 6.7306 |
| 1995 | 6.62678 |
| 1994 | 6.53537 |
| 1993 | 6.48383 |
| 1992 | 6.49871 |
| 1991 | 6.6359 |
| 1990 | 6.87113 |
| 1989 | 7.08916 |
| 1988 | 7.26666 |
| 1987 | 7.41849 |
| 1986 | 7.53119 |
| 1985 | 7.62751 |
| 1984 | 7.78851 |
| 1983 | 8.02326 |
| 1982 | 8.29795 |
| 1981 | 8.61163 |
| 1980 | 8.90623 |
| 1979 | 9.11511 |
| 1978 | 9.25982 |
| 1977 | 9.37193 |
| 1976 | 9.43879 |
| 1975 | 9.44993 |
| 1974 | 9.42528 |
| 1973 | 9.37292 |
| 1972 | 9.26755 |
| 1971 | 9.2962 |
| 1970 | 9.21986 |
| 1969 | 8.85476 |
| 1968 | 8.60001 |
| 1967 | 8.58451 |
| 1966 | 8.45678 |
| 1965 | 8.23193 |
| 1964 | 8.17985 |
| 1963 | 8.09118 |
| 1962 | 7.81776 |
| 1961 | 7.41642 |
| 1960 | 7.18607 |