| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.26187 |
| 2021 | 6.40682 |
| 2020 | 6.5441 |
| 2019 | 6.62698 |
| 2018 | 6.66871 |
| 2017 | 6.70069 |
| 2016 | 6.71267 |
| 2015 | 6.69193 |
| 2014 | 6.65289 |
| 2013 | 6.59492 |
| 2012 | 6.49262 |
| 2011 | 6.47474 |
| 2010 | 6.38057 |
| 2009 | 6.11567 |
| 2008 | 5.93065 |
| 2007 | 5.85537 |
| 2006 | 5.69996 |
| 2005 | 5.51284 |
| 2004 | 5.39847 |
| 2003 | 5.25871 |
| 2002 | 5.07507 |
| 2001 | 4.79915 |
| 2000 | 4.61179 |
| 1999 | 4.55233 |
| 1998 | 4.44036 |
| 1997 | 4.30493 |
| 1996 | 4.27608 |
| 1995 | 4.2534 |
| 1994 | 4.17343 |
| 1993 | 4.17327 |
| 1992 | 4.22824 |
| 1991 | 4.29308 |
| 1990 | 4.43284 |
| 1989 | 4.69397 |
| 1988 | 4.88291 |
| 1987 | 4.98154 |
| 1986 | 5.06095 |
| 1985 | 5.00928 |
| 1984 | 4.8336 |
| 1983 | 4.79848 |
| 1982 | 4.85442 |
| 1981 | 4.81535 |
| 1980 | 4.88022 |
| 1979 | 5.06076 |
| 1978 | 5.02759 |
| 1977 | 4.85148 |
| 1976 | 4.80689 |
| 1975 | 4.74562 |
| 1974 | 4.61226 |
| 1973 | 4.56245 |
| 1972 | 4.64152 |
| 1971 | 4.7291 |
| 1970 | 4.80197 |
| 1969 | 4.85526 |
| 1968 | 4.89121 |
| 1967 | 4.90603 |
| 1966 | 4.85146 |
| 1965 | 4.80331 |
| 1964 | 4.78788 |
| 1963 | 4.70779 |
| 1962 | 4.65739 |
| 1961 | 4.65375 |
| 1960 | 4.604 |