| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 3.45492 |
| 2021 | 3.45798 |
| 2020 | 3.69601 |
| 2019 | 3.43457 |
| 2018 | 3.32387 |
| 2017 | 3.32054 |
| 2016 | 3.42259 |
| 2015 | 3.48143 |
| 2014 | 3.69096 |
| 2013 | 4.03266 |
| 2012 | 4.46171 |
| 2011 | 4.82244 |
| 2010 | 4.90402 |
| 2009 | 4.87577 |
| 2008 | 4.4466 |
| 2007 | 4.07335 |
| 2006 | 4.04134 |
| 2005 | 4.08923 |
| 2004 | 4.01631 |
| 2003 | 3.82716 |
| 2002 | 3.44762 |
| 2001 | 3.12381 |
| 2000 | 3.11224 |
| 1999 | 3.08568 |
| 1998 | 3.20156 |
| 1997 | 3.40556 |
| 1996 | 3.55498 |
| 1995 | 3.86025 |
| 1994 | 4.21526 |
| 1993 | 4.60435 |
| 1992 | 4.97047 |
| 1991 | 4.88343 |
| 1990 | 5.60518 |
| 1989 | 5.87121 |
| 1988 | 6.07128 |
| 1987 | 6.42027 |
| 1986 | 6.62652 |
| 1985 | 6.45322 |
| 1984 | 6.23642 |
| 1983 | 6.32114 |
| 1982 | 6.81406 |
| 1981 | 5.64654 |
| 1980 | 5.15354 |
| 1979 | 4.95199 |
| 1978 | 4.94309 |
| 1977 | 5.15562 |
| 1976 | 5.19107 |
| 1975 | 5.62268 |
| 1974 | 5.95411 |
| 1973 | 5.89387 |
| 1972 | 6.51976 |
| 1971 | 6.94307 |
| 1970 | 8.03274 |
| 1969 | 8.63326 |
| 1968 | 9.26845 |
| 1967 | 9.4178 |
| 1966 | 8.4353 |
| 1965 | 7.58725 |
| 1964 | 8.05128 |
| 1963 | 8.83189 |
| 1962 | 9.33167 |
| 1961 | 9.15603 |
| 1960 | 8.99312 |