| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 3.48109 |
| 2021 | 3.24723 |
| 2020 | 3.08981 |
| 2019 | 3.01621 |
| 2018 | 2.91639 |
| 2017 | 2.81075 |
| 2016 | 2.74782 |
| 2015 | 2.70253 |
| 2014 | 2.65694 |
| 2013 | 2.62675 |
| 2012 | 2.60788 |
| 2011 | 2.59559 |
| 2010 | 2.58943 |
| 2009 | 2.62382 |
| 2008 | 2.66419 |
| 2007 | 2.69995 |
| 2006 | 2.76339 |
| 2005 | 2.81337 |
| 2004 | 2.81649 |
| 2003 | 2.88269 |
| 2002 | 2.96623 |
| 2001 | 2.95911 |
| 2000 | 2.9772 |
| 1999 | 3.04245 |
| 1998 | 3.00341 |
| 1997 | 2.92177 |
| 1996 | 2.91204 |
| 1995 | 2.88665 |
| 1994 | 2.83697 |
| 1993 | 2.83432 |
| 1992 | 2.88178 |
| 1991 | 2.91632 |
| 1990 | 2.93275 |
| 1989 | 2.93493 |
| 1988 | 2.92219 |
| 1987 | 2.89062 |
| 1986 | 2.81936 |
| 1985 | 2.75518 |
| 1984 | 2.71281 |
| 1983 | 2.64978 |
| 1982 | 2.60604 |
| 1981 | 2.5837 |
| 1980 | 2.53582 |
| 1979 | 2.49295 |
| 1978 | 2.48672 |
| 1977 | 2.45767 |
| 1976 | 2.4149 |
| 1975 | 2.39824 |
| 1974 | 2.3703 |
| 1973 | 2.31366 |
| 1972 | 2.24675 |
| 1971 | 2.1979 |
| 1970 | 2.16032 |
| 1969 | 2.1304 |
| 1968 | 2.12022 |
| 1967 | 2.1188 |
| 1966 | 2.0943 |
| 1965 | 2.05378 |
| 1964 | 2.01815 |
| 1963 | 1.98543 |
| 1962 | 1.95507 |
| 1961 | 1.92324 |
| 1960 | 1.88628 |