| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 1.82334 |
| 2021 | 2.02543 |
| 2020 | 2.28799 |
| 2019 | 2.60793 |
| 2018 | 3.05067 |
| 2017 | 2.46594 |
| 2016 | 2.85987 |
| 2015 | 2.9846 |
| 2014 | 2.99007 |
| 2013 | 2.84432 |
| 2012 | 2.99888 |
| 2011 | 2.95682 |
| 2010 | 2.76091 |
| 2009 | 3.01002 |
| 2008 | 2.47368 |
| 2007 | 2.43689 |
| 2006 | 2.81818 |
| 2005 | 3.16778 |
| 2004 | 3.49701 |
| 2003 | 3.57555 |
| 2002 | 3.5195 |
| 2001 | 3.13067 |
| 2000 | 3.18263 |
| 1999 | 3.61737 |
| 1998 | 3.14016 |
| 1997 | 3.36614 |
| 1996 | 3.4934 |
| 1995 | 3.96716 |
| 1994 | 4.10613 |
| 1993 | 3.84016 |
| 1992 | 3.89827 |
| 1991 | 3.60955 |
| 1990 | 3.21285 |
| 1989 | 3.20864 |
| 1988 | 3.63818 |
| 1987 | 3.51828 |
| 1986 | 3.94742 |
| 1985 | 4.50692 |
| 1984 | 5.15787 |
| 1983 | 5.3848 |
| 1982 | 5.479 |
| 1981 | 6.35834 |
| 1980 | 4.41123 |
| 1979 | 3.79436 |
| 1978 | 4.35701 |
| 1977 | 4.67803 |
| 1976 | 5.41053 |
| 1975 | 5.62699 |
| 1974 | 5.9902 |
| 1973 | 6.57949 |
| 1972 | 7.14386 |
| 1971 | - |
| 1970 | - |
| 1969 | - |
| 1968 | - |
| 1967 | - |
| 1966 | - |
| 1965 | - |
| 1964 | - |
| 1963 | - |
| 1962 | - |
| 1961 | - |
| 1960 | - |