| 年份 | 美国  | 
                                                            
|---|---|
| 2023 | - | 
| 2022 | 6.3644 | 
| 2021 | 6.34984 | 
| 2020 | 6.33873 | 
| 2019 | 6.35804 | 
| 2018 | 6.42572 | 
| 2017 | 6.52511 | 
| 2016 | 6.65225 | 
| 2015 | 6.78527 | 
| 2014 | 6.88661 | 
| 2013 | 6.92849 | 
| 2012 | 6.90754 | 
| 2011 | 6.83837 | 
| 2010 | 6.76055 | 
| 2009 | 6.71034 | 
| 2008 | 6.6943 | 
| 2007 | 6.7 | 
| 2006 | 6.73067 | 
| 2005 | 6.78513 | 
| 2004 | 6.83696 | 
| 2003 | 6.85445 | 
| 2002 | 6.81978 | 
| 2001 | 6.72265 | 
| 2000 | 6.58996 | 
| 1999 | 6.48424 | 
| 1998 | 6.4273 | 
| 1997 | 6.42309 | 
| 1996 | 6.52077 | 
| 1995 | 6.72325 | 
| 1994 | 6.9428 | 
| 1993 | 7.13402 | 
| 1992 | 7.29184 | 
| 1991 | 7.39753 | 
| 1990 | 7.48982 | 
| 1989 | 7.65244 | 
| 1988 | 7.8841 | 
| 1987 | 8.14309 | 
| 1986 | 8.4159 | 
| 1985 | 8.67137 | 
| 1984 | 8.85074 | 
| 1983 | 8.97449 | 
| 1982 | 9.07926 | 
| 1981 | 9.1284 | 
| 1980 | 9.12525 | 
| 1979 | 9.10365 | 
| 1978 | 9.05608 | 
| 1977 | 8.95906 | 
| 1976 | 8.97472 | 
| 1975 | 8.88779 | 
| 1974 | 8.54379 | 
| 1973 | 8.30613 | 
| 1972 | 8.29219 | 
| 1971 | 8.16869 | 
| 1970 | 7.95399 | 
| 1969 | 7.90625 | 
| 1968 | 7.81796 | 
| 1967 | 7.53832 | 
| 1966 | 7.13611 | 
| 1965 | 6.894 | 
| 1964 | 6.78302 | 
| 1963 | 6.58873 | 
| 1962 | 6.34674 | 
| 1961 | 6.22156 | 
| 1960 | 6.17375 |