| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.36795 |
| 2021 | 6.52692 |
| 2020 | 6.66387 |
| 2019 | 6.74097 |
| 2018 | 6.78137 |
| 2017 | 6.8167 |
| 2016 | 6.83261 |
| 2015 | 6.82127 |
| 2014 | 6.79597 |
| 2013 | 6.75251 |
| 2012 | 6.6661 |
| 2011 | 6.64879 |
| 2010 | 6.56615 |
| 2009 | 6.31016 |
| 2008 | 6.12617 |
| 2007 | 6.09199 |
| 2006 | 5.97678 |
| 2005 | 5.78162 |
| 2004 | 5.67285 |
| 2003 | 5.53287 |
| 2002 | 5.2916 |
| 2001 | 4.99228 |
| 2000 | 4.80042 |
| 1999 | 4.70108 |
| 1998 | 4.56776 |
| 1997 | 4.42846 |
| 1996 | 4.35731 |
| 1995 | 4.30808 |
| 1994 | 4.25003 |
| 1993 | 4.2108 |
| 1992 | 4.19708 |
| 1991 | 4.21379 |
| 1990 | 4.2522 |
| 1989 | 4.35452 |
| 1988 | 4.46665 |
| 1987 | 4.56668 |
| 1986 | 4.7036 |
| 1985 | 4.81046 |
| 1984 | 4.83709 |
| 1983 | 4.97905 |
| 1982 | 5.14172 |
| 1981 | 5.14437 |
| 1980 | 5.20271 |
| 1979 | 5.35052 |
| 1978 | 5.29273 |
| 1977 | 5.12166 |
| 1976 | 5.08347 |
| 1975 | 5.02331 |
| 1974 | 4.89653 |
| 1973 | 4.85526 |
| 1972 | 4.92967 |
| 1971 | 5.01229 |
| 1970 | 5.07776 |
| 1969 | 5.12035 |
| 1968 | 5.14623 |
| 1967 | 5.14872 |
| 1966 | 5.07995 |
| 1965 | 5.01538 |
| 1964 | 4.97586 |
| 1963 | 4.87934 |
| 1962 | 4.81991 |
| 1961 | 4.81042 |
| 1960 | 4.753 |