| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 0.631847 |
| 2021 | 0.409259 |
| 2020 | 1.21265 |
| 2019 | 0.701868 |
| 2018 | 0.767437 |
| 2017 | 0.871785 |
| 2016 | 0.958268 |
| 2015 | 0.966675 |
| 2014 | 0.959431 |
| 2013 | 0.91451 |
| 2012 | 0.949566 |
| 2011 | 0.939505 |
| 2010 | 1.03534 |
| 2009 | 1.08529 |
| 2008 | 1.15619 |
| 2007 | 1.16307 |
| 2006 | 1.17797 |
| 2005 | 1.13588 |
| 2004 | 1.14389 |
| 2003 | 1.07836 |
| 2002 | 1.14842 |
| 2001 | 1.21338 |
| 2000 | 1.51201 |
| 1999 | 1.61296 |
| 1998 | 1.63763 |
| 1997 | 1.68198 |
| 1996 | 1.65278 |
| 1995 | 1.68647 |
| 1994 | 1.73098 |
| 1993 | 1.83118 |
| 1992 | 1.90863 |
| 1991 | 1.86738 |
| 1990 | 1.41026 |
| 1989 | 1.14036 |
| 1988 | 1.10702 |
| 1987 | 1.09324 |
| 1986 | 1.12532 |
| 1985 | 1.08769 |
| 1984 | 1.06779 |
| 1983 | 1.11946 |
| 1982 | 1.15882 |
| 1981 | 1.18734 |
| 1980 | 1.02199 |
| 1979 | 1.11715 |
| 1978 | 1.07179 |
| 1977 | 1.01935 |
| 1976 | 0.963796 |
| 1975 | 0.999565 |
| 1974 | 0.92724 |
| 1973 | 0.968059 |
| 1972 | 1.08411 |
| 1971 | 1.27928 |
| 1970 | 1.53115 |
| 1969 | 1.46799 |
| 1968 | 1.49576 |
| 1967 | 1.59282 |
| 1966 | 1.66556 |
| 1965 | 1.76765 |
| 1964 | 1.91344 |
| 1963 | 1.97056 |
| 1962 | 2.07649 |
| 1961 | 2.20057 |
| 1960 | - |