| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.20558 |
| 2021 | 6.21776 |
| 2020 | 6.1931 |
| 2019 | 6.14587 |
| 2018 | 6.07481 |
| 2017 | 5.96604 |
| 2016 | 5.94021 |
| 2015 | 5.84095 |
| 2014 | 5.58507 |
| 2013 | 5.4109 |
| 2012 | 5.3453 |
| 2011 | 5.21348 |
| 2010 | 5.04934 |
| 2009 | 4.94135 |
| 2008 | 4.80651 |
| 2007 | 4.63502 |
| 2006 | 4.38114 |
| 2005 | 4.20469 |
| 2004 | 4.14155 |
| 2003 | 4.02862 |
| 2002 | 3.89549 |
| 2001 | 3.85977 |
| 2000 | 3.83431 |
| 1999 | 3.76068 |
| 1998 | 3.75255 |
| 1997 | 3.78875 |
| 1996 | 3.83331 |
| 1995 | 3.94391 |
| 1994 | 4.16235 |
| 1993 | 4.32149 |
| 1992 | 4.41183 |
| 1991 | 4.48612 |
| 1990 | 4.43885 |
| 1989 | 4.28135 |
| 1988 | 4.24088 |
| 1987 | 4.28189 |
| 1986 | 4.24084 |
| 1985 | 4.28417 |
| 1984 | 4.41762 |
| 1983 | 4.38249 |
| 1982 | 4.26069 |
| 1981 | 4.24787 |
| 1980 | 4.19689 |
| 1979 | 4.07454 |
| 1978 | 4.02054 |
| 1977 | 4.07717 |
| 1976 | 4.13111 |
| 1975 | 4.16429 |
| 1974 | 4.1815 |
| 1973 | 4.18065 |
| 1972 | 4.16433 |
| 1971 | 4.09904 |
| 1970 | 4.04414 |
| 1969 | 4.01091 |
| 1968 | 3.91162 |
| 1967 | 3.82905 |
| 1966 | 3.78381 |
| 1965 | 3.71056 |
| 1964 | 3.67396 |
| 1963 | 3.74185 |
| 1962 | 3.78988 |
| 1961 | 3.80251 |
| 1960 | 3.87152 |