| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 0.651673 |
| 2021 | 0.618595 |
| 2020 | 0.503466 |
| 2019 | 0.43511 |
| 2018 | 0.627411 |
| 2017 | 6.803 |
| 2016 | 0.574954 |
| 2015 | 0.580888 |
| 2014 | 0.565746 |
| 2013 | 0.659561 |
| 2012 | 0.516844 |
| 2011 | 0.371626 |
| 2010 | 0.610642 |
| 2009 | 0.9083 |
| 2008 | 1.07984 |
| 2007 | 0.979869 |
| 2006 | 1.081 |
| 2005 | 1.06862 |
| 2004 | 1.19587 |
| 2003 | 1.14294 |
| 2002 | 1.33137 |
| 2001 | 1.35359 |
| 2000 | 1.37536 |
| 1999 | 1.45808 |
| 1998 | 1.33998 |
| 1997 | 1.1919 |
| 1996 | 1.12351 |
| 1995 | 1.02873 |
| 1994 | 1.14473 |
| 1993 | 1.01639 |
| 1992 | 0.970025 |
| 1991 | 0.965767 |
| 1990 | 1.06113 |
| 1989 | 0.837991 |
| 1988 | 0.790592 |
| 1987 | 0.825438 |
| 1986 | 0.848045 |
| 1985 | 0.813696 |
| 1984 | 0.839459 |
| 1983 | 0.92757 |
| 1982 | 1.21284 |
| 1981 | 1.06285 |
| 1980 | 1.17425 |
| 1979 | 1.10925 |
| 1978 | 1.2723 |
| 1977 | 1.97678 |
| 1976 | 1.68135 |
| 1975 | 1.5842 |
| 1974 | 1.82264 |
| 1973 | 2.02099 |
| 1972 | 2.54804 |
| 1971 | - |
| 1970 | - |
| 1969 | - |
| 1968 | - |
| 1967 | - |
| 1966 | - |
| 1965 | - |
| 1964 | - |
| 1963 | - |
| 1962 | - |
| 1961 | - |
| 1960 | - |