| 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 |