| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.83149 |
| 2021 | 6.81541 |
| 2020 | 6.80521 |
| 2019 | 6.79083 |
| 2018 | 6.75565 |
| 2017 | 6.69078 |
| 2016 | 6.59604 |
| 2015 | 6.48774 |
| 2014 | 6.40442 |
| 2013 | 6.36515 |
| 2012 | 6.37725 |
| 2011 | 6.47699 |
| 2010 | 6.66149 |
| 2009 | 6.85639 |
| 2008 | 7.01821 |
| 2007 | 7.13416 |
| 2006 | 7.18908 |
| 2005 | 7.22467 |
| 2004 | 7.318 |
| 2003 | 7.4813 |
| 2002 | 7.67387 |
| 2001 | 7.88667 |
| 2000 | 8.10194 |
| 1999 | 8.27368 |
| 1998 | 8.40302 |
| 1997 | 8.50593 |
| 1996 | 8.5699 |
| 1995 | 8.58433 |
| 1994 | 8.56666 |
| 1993 | 8.51686 |
| 1992 | 8.40118 |
| 1991 | 8.39992 |
| 1990 | 8.28604 |
| 1989 | 7.92564 |
| 1988 | 7.6691 |
| 1987 | 7.54708 |
| 1986 | 7.30796 |
| 1985 | 7.05363 |
| 1984 | 6.95239 |
| 1983 | 6.8368 |
| 1982 | 6.64411 |
| 1981 | 6.31611 |
| 1980 | 6.10967 |
| 1979 | 6.05834 |
| 1978 | 5.8969 |
| 1977 | 5.67629 |
| 1976 | 5.61648 |
| 1975 | 5.58648 |
| 1974 | 5.47949 |
| 1973 | 5.5073 |
| 1972 | 5.61649 |
| 1971 | 5.71502 |
| 1970 | 5.91193 |
| 1969 | 6.28243 |
| 1968 | 6.52769 |
| 1967 | 6.629 |
| 1966 | 6.73487 |
| 1965 | 6.70725 |
| 1964 | 6.52118 |
| 1963 | 6.54414 |
| 1962 | 6.68976 |
| 1961 | 6.65154 |
| 1960 | 6.7302 |