| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.37924 |
| 2021 | 6.30325 |
| 2020 | 6.20625 |
| 2019 | 6.12471 |
| 2018 | 6.07687 |
| 2017 | 6.07462 |
| 2016 | 6.1606 |
| 2015 | 6.32902 |
| 2014 | 6.49809 |
| 2013 | 6.62996 |
| 2012 | 6.7234 |
| 2011 | 6.76384 |
| 2010 | 6.79432 |
| 2009 | 6.89443 |
| 2008 | 7.06889 |
| 2007 | 7.26376 |
| 2006 | 7.46465 |
| 2005 | 7.64894 |
| 2004 | 7.76727 |
| 2003 | 7.83306 |
| 2002 | 7.88675 |
| 2001 | 7.91817 |
| 2000 | 7.91312 |
| 1999 | 7.88994 |
| 1998 | 7.84684 |
| 1997 | 7.75571 |
| 1996 | 7.75265 |
| 1995 | 7.6677 |
| 1994 | 7.37343 |
| 1993 | 7.17084 |
| 1992 | 7.15417 |
| 1991 | 7.04352 |
| 1990 | 6.84421 |
| 1989 | 6.76228 |
| 1988 | 6.64004 |
| 1987 | 6.38421 |
| 1986 | 6.04047 |
| 1985 | 5.83157 |
| 1984 | 5.733 |
| 1983 | 5.57204 |
| 1982 | 5.38902 |
| 1981 | 5.2872 |
| 1980 | 5.22573 |
| 1979 | 5.15671 |
| 1978 | 5.12246 |
| 1977 | 5.12767 |
| 1976 | 5.14101 |
| 1975 | 5.17863 |
| 1974 | 5.32029 |
| 1973 | 5.44147 |
| 1972 | 5.52417 |
| 1971 | 5.6891 |
| 1970 | 5.85328 |
| 1969 | 5.91063 |
| 1968 | 6.11078 |
| 1967 | 6.34041 |
| 1966 | 6.34424 |
| 1965 | 6.40448 |
| 1964 | 6.59241 |
| 1963 | 6.55047 |
| 1962 | 6.40177 |
| 1961 | 6.45191 |
| 1960 | 6.48111 |