| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 4.58959 |
| 2021 | 4.46689 |
| 2020 | 4.3796 |
| 2019 | 4.34253 |
| 2018 | 4.33969 |
| 2017 | 4.35128 |
| 2016 | 4.36691 |
| 2015 | 4.38027 |
| 2014 | 4.39605 |
| 2013 | 4.40923 |
| 2012 | 4.42621 |
| 2011 | 4.43601 |
| 2010 | 4.42821 |
| 2009 | 4.38214 |
| 2008 | 4.34011 |
| 2007 | 4.31481 |
| 2006 | 4.25973 |
| 2005 | 4.22292 |
| 2004 | 4.20701 |
| 2003 | 4.13799 |
| 2002 | 4.07858 |
| 2001 | 4.0634 |
| 2000 | 4.02918 |
| 1999 | 3.99073 |
| 1998 | 3.97863 |
| 1997 | 3.98513 |
| 1996 | 3.97519 |
| 1995 | 3.95512 |
| 1994 | 3.92671 |
| 1993 | 3.88818 |
| 1992 | 3.82949 |
| 1991 | 3.74727 |
| 1990 | 3.66739 |
| 1989 | 3.59829 |
| 1988 | 3.53371 |
| 1987 | 3.4616 |
| 1986 | 3.39806 |
| 1985 | 3.3322 |
| 1984 | 3.26171 |
| 1983 | 3.21036 |
| 1982 | 3.13156 |
| 1981 | 3.0522 |
| 1980 | 2.99436 |
| 1979 | 2.93037 |
| 1978 | 2.85484 |
| 1977 | 2.76513 |
| 1976 | 2.68315 |
| 1975 | 2.59194 |
| 1974 | 2.50101 |
| 1973 | 2.4382 |
| 1972 | 2.38995 |
| 1971 | 2.33091 |
| 1970 | 2.26076 |
| 1969 | 2.18744 |
| 1968 | 2.11625 |
| 1967 | 2.05488 |
| 1966 | 2.00335 |
| 1965 | 1.95123 |
| 1964 | 1.89586 |
| 1963 | 1.84789 |
| 1962 | 1.80853 |
| 1961 | 1.76925 |
| 1960 | 1.72744 |