| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 3.26411 |
| 2021 | 3.34186 |
| 2020 | 3.15187 |
| 2019 | 3.65133 |
| 2018 | 2.59783 |
| 2017 | 1.70702 |
| 2016 | 1.83884 |
| 2015 | 1.86338 |
| 2014 | 1.94676 |
| 2013 | 2.00866 |
| 2012 | 2.11106 |
| 2011 | 2.14314 |
| 2010 | 2.21956 |
| 2009 | 2.01708 |
| 2008 | 1.9267 |
| 2007 | 1.76198 |
| 2006 | 1.70785 |
| 2005 | 1.81897 |
| 2004 | 1.99817 |
| 2003 | 1.99454 |
| 2002 | 1.8487 |
| 2001 | 1.6446 |
| 2000 | 1.50418 |
| 1999 | 1.56168 |
| 1998 | 1.64753 |
| 1997 | 1.77711 |
| 1996 | 2.04524 |
| 1995 | 2.31727 |
| 1994 | 2.61703 |
| 1993 | 2.67838 |
| 1992 | 2.6851 |
| 1991 | 2.52226 |
| 1990 | 2.69365 |
| 1989 | 2.70426 |
| 1988 | 2.94134 |
| 1987 | 2.84806 |
| 1986 | 2.88336 |
| 1985 | 2.70324 |
| 1984 | 2.79369 |
| 1983 | 2.31878 |
| 1982 | 2.2245 |
| 1981 | 2.02733 |
| 1980 | 2.14226 |
| 1979 | 2.43833 |
| 1978 | 2.51262 |
| 1977 | 2.22424 |
| 1976 | 3.06401 |
| 1975 | 2.27622 |
| 1974 | 1.88607 |
| 1973 | 1.99573 |
| 1972 | 2.22898 |
| 1971 | - |
| 1970 | - |
| 1969 | - |
| 1968 | - |
| 1967 | - |
| 1966 | - |
| 1965 | - |
| 1964 | - |
| 1963 | - |
| 1962 | - |
| 1961 | - |
| 1960 | - |