| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 6.27526 |
| 2021 | 6.30333 |
| 2020 | 6.30858 |
| 2019 | 6.36976 |
| 2018 | 6.50293 |
| 2017 | 6.65819 |
| 2016 | 6.82614 |
| 2015 | 6.98182 |
| 2014 | 7.08116 |
| 2013 | 7.13824 |
| 2012 | 7.18761 |
| 2011 | 7.2195 |
| 2010 | 7.21391 |
| 2009 | 7.17964 |
| 2008 | 7.12364 |
| 2007 | 7.02368 |
| 2006 | 7.00291 |
| 2005 | 6.91948 |
| 2004 | 6.65246 |
| 2003 | 6.46044 |
| 2002 | 6.42556 |
| 2001 | 6.30544 |
| 2000 | 6.11118 |
| 1999 | 6.01827 |
| 1998 | 5.89456 |
| 1997 | 5.6525 |
| 1996 | 5.33467 |
| 1995 | 5.13943 |
| 1994 | 5.04642 |
| 1993 | 4.90527 |
| 1992 | 4.7536 |
| 1991 | 4.67865 |
| 1990 | 4.63148 |
| 1989 | 4.57494 |
| 1988 | 4.53234 |
| 1987 | 4.51507 |
| 1986 | 4.53501 |
| 1985 | 4.58683 |
| 1984 | 4.70624 |
| 1983 | 4.82391 |
| 1982 | 4.9288 |
| 1981 | 5.08067 |
| 1980 | 5.19501 |
| 1979 | 5.20731 |
| 1978 | 5.35457 |
| 1977 | 5.54669 |
| 1976 | 5.55336 |
| 1975 | 5.59866 |
| 1974 | 5.74875 |
| 1973 | 5.69323 |
| 1972 | 5.52666 |
| 1971 | 5.51325 |
| 1970 | 5.48248 |
| 1969 | 5.38034 |
| 1968 | 5.34767 |
| 1967 | 5.40937 |
| 1966 | 5.46964 |
| 1965 | 5.5134 |
| 1964 | 5.5371 |
| 1963 | 5.54986 |
| 1962 | 5.53733 |
| 1961 | 5.44595 |
| 1960 | 5.36895 |