| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 4.7149 |
| 2021 | 4.61266 |
| 2020 | 4.45845 |
| 2019 | 4.36422 |
| 2018 | 4.24199 |
| 2017 | 4.04434 |
| 2016 | 3.79915 |
| 2015 | 3.64051 |
| 2014 | 3.56406 |
| 2013 | 3.45466 |
| 2012 | 3.33907 |
| 2011 | 3.27429 |
| 2010 | 3.22176 |
| 2009 | 3.16158 |
| 2008 | 3.12217 |
| 2007 | 3.10389 |
| 2006 | 3.10043 |
| 2005 | 3.10509 |
| 2004 | 3.15536 |
| 2003 | 3.20906 |
| 2002 | 3.25439 |
| 2001 | 3.33551 |
| 2000 | 3.40273 |
| 1999 | 3.4087 |
| 1998 | 3.48537 |
| 1997 | 3.58616 |
| 1996 | 3.58688 |
| 1995 | 3.62219 |
| 1994 | 3.71198 |
| 1993 | 3.67119 |
| 1992 | 3.57515 |
| 1991 | 3.56745 |
| 1990 | 3.54288 |
| 1989 | 3.48693 |
| 1988 | 3.4799 |
| 1987 | 3.52725 |
| 1986 | 3.56826 |
| 1985 | 3.59738 |
| 1984 | 3.60612 |
| 1983 | 3.59307 |
| 1982 | 3.55699 |
| 1981 | 3.47393 |
| 1980 | 3.39214 |
| 1979 | 3.31926 |
| 1978 | 3.21215 |
| 1977 | 3.1304 |
| 1976 | 3.08224 |
| 1975 | 3.02468 |
| 1974 | 2.99685 |
| 1973 | 3.02745 |
| 1972 | 3.03768 |
| 1971 | 3.0355 |
| 1970 | 3.07173 |
| 1969 | 3.09587 |
| 1968 | 3.06589 |
| 1967 | 3.00609 |
| 1966 | 2.98167 |
| 1965 | 2.97133 |
| 1964 | 2.94203 |
| 1963 | 2.92794 |
| 1962 | 2.93002 |
| 1961 | 2.90199 |
| 1960 | 2.85345 |