| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.78265 |
| 2021 | 6.74716 |
| 2020 | 6.74042 |
| 2019 | 6.75167 |
| 2018 | 6.75938 |
| 2017 | 6.77265 |
| 2016 | 6.79641 |
| 2015 | 6.82591 |
| 2014 | 6.88063 |
| 2013 | 6.97709 |
| 2012 | 7.11198 |
| 2011 | 7.27855 |
| 2010 | 7.43395 |
| 2009 | 7.5456 |
| 2008 | 7.61099 |
| 2007 | 7.62743 |
| 2006 | 7.61174 |
| 2005 | 7.58042 |
| 2004 | 7.53916 |
| 2003 | 7.50802 |
| 2002 | 7.48807 |
| 2001 | 7.47758 |
| 2000 | 7.48443 |
| 1999 | 7.50507 |
| 1998 | 7.51433 |
| 1997 | 7.48009 |
| 1996 | 7.39767 |
| 1995 | 7.29057 |
| 1994 | 7.19565 |
| 1993 | 7.14579 |
| 1992 | 7.16856 |
| 1991 | 7.31893 |
| 1990 | 7.5706 |
| 1989 | 7.79894 |
| 1988 | 7.9806 |
| 1987 | 8.13206 |
| 1986 | 8.23446 |
| 1985 | 8.31055 |
| 1984 | 8.4592 |
| 1983 | 8.70331 |
| 1982 | 8.99711 |
| 1981 | 9.33104 |
| 1980 | 9.64956 |
| 1979 | 9.87114 |
| 1978 | 10.0132 |
| 1977 | 10.1263 |
| 1976 | 10.1893 |
| 1975 | 10.18 |
| 1974 | 10.1302 |
| 1973 | 10.0563 |
| 1972 | 9.93036 |
| 1971 | 9.96874 |
| 1970 | 9.85393 |
| 1969 | 9.43123 |
| 1968 | 9.14225 |
| 1967 | 9.03309 |
| 1966 | 8.81616 |
| 1965 | 8.59179 |
| 1964 | 8.50493 |
| 1963 | 8.35053 |
| 1962 | 8.08299 |
| 1961 | 7.63192 |
| 1960 | 7.35121 |