| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.36864 |
| 2021 | 6.39037 |
| 2020 | 6.37403 |
| 2019 | 6.40058 |
| 2018 | 6.49633 |
| 2017 | 6.62158 |
| 2016 | 6.76931 |
| 2015 | 6.91801 |
| 2014 | 7.01997 |
| 2013 | 7.08038 |
| 2012 | 7.13014 |
| 2011 | 7.16314 |
| 2010 | 7.15939 |
| 2009 | 7.12625 |
| 2008 | 7.06709 |
| 2007 | 6.95824 |
| 2006 | 6.94601 |
| 2005 | 6.85361 |
| 2004 | 6.57325 |
| 2003 | 6.37636 |
| 2002 | 6.29199 |
| 2001 | 6.12203 |
| 2000 | 5.93103 |
| 1999 | 5.82911 |
| 1998 | 5.70572 |
| 1997 | 5.5285 |
| 1996 | 5.23658 |
| 1995 | 5.04733 |
| 1994 | 4.99815 |
| 1993 | 4.87782 |
| 1992 | 4.72949 |
| 1991 | 4.70339 |
| 1990 | 4.68841 |
| 1989 | 4.60991 |
| 1988 | 4.61382 |
| 1987 | 4.67358 |
| 1986 | 4.74569 |
| 1985 | 4.91027 |
| 1984 | 5.20776 |
| 1983 | 5.41217 |
| 1982 | 5.51749 |
| 1981 | 5.6088 |
| 1980 | 5.55282 |
| 1979 | 5.34923 |
| 1978 | 5.30891 |
| 1977 | 5.38396 |
| 1976 | 5.33873 |
| 1975 | 5.38819 |
| 1974 | 5.57594 |
| 1973 | 5.54515 |
| 1972 | 5.36892 |
| 1971 | 5.35358 |
| 1970 | 5.33255 |
| 1969 | 5.22857 |
| 1968 | 5.1875 |
| 1967 | 5.25349 |
| 1966 | 5.30814 |
| 1965 | 5.34304 |
| 1964 | 5.36838 |
| 1963 | 5.39427 |
| 1962 | 5.40888 |
| 1961 | 5.35168 |
| 1960 | 5.30267 |