| 年份 | 美国  | 
                                                            
|---|---|
| 2023 | - | 
| 2022 | 6.36795 | 
| 2021 | 6.52692 | 
| 2020 | 6.66387 | 
| 2019 | 6.74097 | 
| 2018 | 6.78137 | 
| 2017 | 6.8167 | 
| 2016 | 6.83261 | 
| 2015 | 6.82127 | 
| 2014 | 6.79597 | 
| 2013 | 6.75251 | 
| 2012 | 6.6661 | 
| 2011 | 6.64879 | 
| 2010 | 6.56615 | 
| 2009 | 6.31016 | 
| 2008 | 6.12617 | 
| 2007 | 6.09199 | 
| 2006 | 5.97678 | 
| 2005 | 5.78162 | 
| 2004 | 5.67285 | 
| 2003 | 5.53287 | 
| 2002 | 5.2916 | 
| 2001 | 4.99228 | 
| 2000 | 4.80042 | 
| 1999 | 4.70108 | 
| 1998 | 4.56776 | 
| 1997 | 4.42846 | 
| 1996 | 4.35731 | 
| 1995 | 4.30808 | 
| 1994 | 4.25003 | 
| 1993 | 4.2108 | 
| 1992 | 4.19708 | 
| 1991 | 4.21379 | 
| 1990 | 4.2522 | 
| 1989 | 4.35452 | 
| 1988 | 4.46665 | 
| 1987 | 4.56668 | 
| 1986 | 4.7036 | 
| 1985 | 4.81046 | 
| 1984 | 4.83709 | 
| 1983 | 4.97905 | 
| 1982 | 5.14172 | 
| 1981 | 5.14437 | 
| 1980 | 5.20271 | 
| 1979 | 5.35052 | 
| 1978 | 5.29273 | 
| 1977 | 5.12166 | 
| 1976 | 5.08347 | 
| 1975 | 5.02331 | 
| 1974 | 4.89653 | 
| 1973 | 4.85526 | 
| 1972 | 4.92967 | 
| 1971 | 5.01229 | 
| 1970 | 5.07776 | 
| 1969 | 5.12035 | 
| 1968 | 5.14623 | 
| 1967 | 5.14872 | 
| 1966 | 5.07995 | 
| 1965 | 5.01538 | 
| 1964 | 4.97586 | 
| 1963 | 4.87934 | 
| 1962 | 4.81991 | 
| 1961 | 4.81042 | 
| 1960 | 4.753 |