| 年份 | 美国  | 
                                                            
|---|---|
| 2023 | - | 
| 2022 | 6.4352 | 
| 2021 | 6.43917 | 
| 2020 | 6.41389 | 
| 2019 | 6.37965 | 
| 2018 | 6.32756 | 
| 2017 | 6.23906 | 
| 2016 | 6.22281 | 
| 2015 | 6.14027 | 
| 2014 | 5.89149 | 
| 2013 | 5.71627 | 
| 2012 | 5.68564 | 
| 2011 | 5.58381 | 
| 2010 | 5.40603 | 
| 2009 | 5.30186 | 
| 2008 | 5.16723 | 
| 2007 | 4.94083 | 
| 2006 | 4.6613 | 
| 2005 | 4.48082 | 
| 2004 | 4.38508 | 
| 2003 | 4.25332 | 
| 2002 | 4.1152 | 
| 2001 | 4.04212 | 
| 2000 | 3.99298 | 
| 1999 | 3.93668 | 
| 1998 | 3.89364 | 
| 1997 | 3.87183 | 
| 1996 | 3.87986 | 
| 1995 | 3.90829 | 
| 1994 | 3.99508 | 
| 1993 | 4.09384 | 
| 1992 | 4.19044 | 
| 1991 | 4.32269 | 
| 1990 | 4.42196 | 
| 1989 | 4.44315 | 
| 1988 | 4.56153 | 
| 1987 | 4.70131 | 
| 1986 | 4.69572 | 
| 1985 | 4.73105 | 
| 1984 | 4.83619 | 
| 1983 | 4.77672 | 
| 1982 | 4.6591 | 
| 1981 | 4.66143 | 
| 1980 | 4.62047 | 
| 1979 | 4.50665 | 
| 1978 | 4.46457 | 
| 1977 | 4.52901 | 
| 1976 | 4.59356 | 
| 1975 | 4.63697 | 
| 1974 | 4.66167 | 
| 1973 | 4.66078 | 
| 1972 | 4.62343 | 
| 1971 | 4.51815 | 
| 1970 | 4.42244 | 
| 1969 | 4.35443 | 
| 1968 | 4.23706 | 
| 1967 | 4.15303 | 
| 1966 | 4.11451 | 
| 1965 | 4.04071 | 
| 1964 | 4.00105 | 
| 1963 | 4.06795 | 
| 1962 | 4.11681 | 
| 1961 | 4.13437 | 
| 1960 | 4.20667 |