| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.3644 |
| 2021 | 6.34984 |
| 2020 | 6.33873 |
| 2019 | 6.35804 |
| 2018 | 6.42572 |
| 2017 | 6.52511 |
| 2016 | 6.65225 |
| 2015 | 6.78527 |
| 2014 | 6.88661 |
| 2013 | 6.92849 |
| 2012 | 6.90754 |
| 2011 | 6.83837 |
| 2010 | 6.76055 |
| 2009 | 6.71034 |
| 2008 | 6.6943 |
| 2007 | 6.7 |
| 2006 | 6.73067 |
| 2005 | 6.78513 |
| 2004 | 6.83696 |
| 2003 | 6.85445 |
| 2002 | 6.81978 |
| 2001 | 6.72265 |
| 2000 | 6.58996 |
| 1999 | 6.48424 |
| 1998 | 6.4273 |
| 1997 | 6.42309 |
| 1996 | 6.52077 |
| 1995 | 6.72325 |
| 1994 | 6.9428 |
| 1993 | 7.13402 |
| 1992 | 7.29184 |
| 1991 | 7.39753 |
| 1990 | 7.48982 |
| 1989 | 7.65244 |
| 1988 | 7.8841 |
| 1987 | 8.14309 |
| 1986 | 8.4159 |
| 1985 | 8.67137 |
| 1984 | 8.85074 |
| 1983 | 8.97449 |
| 1982 | 9.07926 |
| 1981 | 9.1284 |
| 1980 | 9.12525 |
| 1979 | 9.10365 |
| 1978 | 9.05608 |
| 1977 | 8.95906 |
| 1976 | 8.97472 |
| 1975 | 8.88779 |
| 1974 | 8.54379 |
| 1973 | 8.30613 |
| 1972 | 8.29219 |
| 1971 | 8.16869 |
| 1970 | 7.95399 |
| 1969 | 7.90625 |
| 1968 | 7.81796 |
| 1967 | 7.53832 |
| 1966 | 7.13611 |
| 1965 | 6.894 |
| 1964 | 6.78302 |
| 1963 | 6.58873 |
| 1962 | 6.34674 |
| 1961 | 6.22156 |
| 1960 | 6.17375 |