| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.74146 |
| 2021 | 6.72192 |
| 2020 | 6.70657 |
| 2019 | 6.72958 |
| 2018 | 6.81124 |
| 2017 | 6.93198 |
| 2016 | 7.08311 |
| 2015 | 7.23675 |
| 2014 | 7.35455 |
| 2013 | 7.40925 |
| 2012 | 7.39672 |
| 2011 | 7.3355 |
| 2010 | 7.26706 |
| 2009 | 7.23289 |
| 2008 | 7.24158 |
| 2007 | 7.27164 |
| 2006 | 7.31096 |
| 2005 | 7.34846 |
| 2004 | 7.36924 |
| 2003 | 7.36291 |
| 2002 | 7.32264 |
| 2001 | 7.23053 |
| 2000 | 7.10412 |
| 1999 | 6.99822 |
| 1998 | 6.93786 |
| 1997 | 6.93445 |
| 1996 | 7.0355 |
| 1995 | 7.24338 |
| 1994 | 7.4748 |
| 1993 | 7.69071 |
| 1992 | 7.8726 |
| 1991 | 7.99045 |
| 1990 | 8.0829 |
| 1989 | 8.22331 |
| 1988 | 8.42466 |
| 1987 | 8.66877 |
| 1986 | 8.93525 |
| 1985 | 9.18831 |
| 1984 | 9.3663 |
| 1983 | 9.47985 |
| 1982 | 9.57502 |
| 1981 | 9.62017 |
| 1980 | 9.60564 |
| 1979 | 9.57286 |
| 1978 | 9.51941 |
| 1977 | 9.40576 |
| 1976 | 9.42135 |
| 1975 | 9.27876 |
| 1974 | 8.84501 |
| 1973 | 8.52588 |
| 1972 | 8.34 |
| 1971 | 7.99264 |
| 1970 | 7.61587 |
| 1969 | 7.42562 |
| 1968 | 7.26285 |
| 1967 | 7.10153 |
| 1966 | 6.84527 |
| 1965 | 6.67967 |
| 1964 | 6.62016 |
| 1963 | 6.42717 |
| 1962 | 6.17831 |
| 1961 | 6.13083 |
| 1960 | 6.12105 |