| 年份 | 美国  | 
                                                            
|---|---|
| 2023 | - | 
| 2022 | 6.56825 | 
| 2021 | 6.46867 | 
| 2020 | 6.3512 | 
| 2019 | 6.25654 | 
| 2018 | 6.20221 | 
| 2017 | 6.19689 | 
| 2016 | 6.28134 | 
| 2015 | 6.44913 | 
| 2014 | 6.61804 | 
| 2013 | 6.75229 | 
| 2012 | 6.84737 | 
| 2011 | 6.88165 | 
| 2010 | 6.9003 | 
| 2009 | 6.9805 | 
| 2008 | 7.12832 | 
| 2007 | 7.30313 | 
| 2006 | 7.49523 | 
| 2005 | 7.68456 | 
| 2004 | 7.82031 | 
| 2003 | 7.90807 | 
| 2002 | 7.98152 | 
| 2001 | 8.03115 | 
| 2000 | 8.0406 | 
| 1999 | 8.02739 | 
| 1998 | 7.98834 | 
| 1997 | 7.88902 | 
| 1996 | 7.89613 | 
| 1995 | 7.79959 | 
| 1994 | 7.48305 | 
| 1993 | 7.26745 | 
| 1992 | 7.19163 | 
| 1991 | 7.01847 | 
| 1990 | 6.8101 | 
| 1989 | 6.70292 | 
| 1988 | 6.56541 | 
| 1987 | 6.37434 | 
| 1986 | 6.05579 | 
| 1985 | 5.84827 | 
| 1984 | 5.794 | 
| 1983 | 5.65281 | 
| 1982 | 5.46689 | 
| 1981 | 5.41554 | 
| 1980 | 5.38604 | 
| 1979 | 5.2872 | 
| 1978 | 5.31025 | 
| 1977 | 5.41251 | 
| 1976 | 5.49988 | 
| 1975 | 5.68111 | 
| 1974 | 6.03335 | 
| 1973 | 6.25384 | 
| 1972 | 6.33241 | 
| 1971 | 6.42464 | 
| 1970 | 6.38878 | 
| 1969 | 6.19285 | 
| 1968 | 6.18758 | 
| 1967 | 6.28684 | 
| 1966 | 6.20571 | 
| 1965 | 6.24191 | 
| 1964 | 6.45777 | 
| 1963 | 6.42996 | 
| 1962 | 6.25175 | 
| 1961 | 6.28773 | 
| 1960 | 6.31782 |