| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.11236 |
| 2021 | 6.24197 |
| 2020 | 6.34398 |
| 2019 | 6.39514 |
| 2018 | 6.4008 |
| 2017 | 6.37667 |
| 2016 | 6.34711 |
| 2015 | 6.35047 |
| 2014 | 6.373 |
| 2013 | 6.39026 |
| 2012 | 6.4125 |
| 2011 | 6.44075 |
| 2010 | 6.47245 |
| 2009 | 6.52164 |
| 2008 | 6.59696 |
| 2007 | 6.69941 |
| 2006 | 6.82004 |
| 2005 | 6.94037 |
| 2004 | 7.03881 |
| 2003 | 7.08981 |
| 2002 | 7.08777 |
| 2001 | 7.05147 |
| 2000 | 6.99871 |
| 1999 | 6.93457 |
| 1998 | 6.88106 |
| 1997 | 6.85347 |
| 1996 | 6.84649 |
| 1995 | 6.85173 |
| 1994 | 6.86128 |
| 1993 | 6.84956 |
| 1992 | 6.79683 |
| 1991 | 6.70855 |
| 1990 | 6.60338 |
| 1989 | 6.51939 |
| 1988 | 6.47578 |
| 1987 | 6.49189 |
| 1986 | 6.61551 |
| 1985 | 6.83769 |
| 1984 | 7.06345 |
| 1983 | 7.26557 |
| 1982 | 7.44834 |
| 1981 | 7.57075 |
| 1980 | 7.67657 |
| 1979 | 7.86698 |
| 1978 | 8.15231 |
| 1977 | 8.4851 |
| 1976 | 8.82851 |
| 1975 | 9.14069 |
| 1974 | 9.36626 |
| 1973 | 9.51421 |
| 1972 | 9.63554 |
| 1971 | 9.71739 |
| 1970 | 9.74954 |
| 1969 | 9.7537 |
| 1968 | 9.72642 |
| 1967 | 9.64041 |
| 1966 | 9.68438 |
| 1965 | 9.62228 |
| 1964 | 9.27402 |
| 1963 | 9.05482 |
| 1962 | 9.09303 |
| 1961 | 9.01376 |
| 1960 | 8.83181 |