| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 26.3865 |
| 2021 | 25.6291 |
| 2020 | 24.8576 |
| 2019 | 24.1198 |
| 2018 | 23.4395 |
| 2017 | 22.8701 |
| 2016 | 22.1792 |
| 2015 | 21.586 |
| 2014 | 21.2753 |
| 2013 | 20.8663 |
| 2012 | 20.3441 |
| 2011 | 19.8104 |
| 2010 | 19.4146 |
| 2009 | 19.1493 |
| 2008 | 18.8573 |
| 2007 | 18.6053 |
| 2006 | 18.4967 |
| 2005 | 18.44 |
| 2004 | 18.3983 |
| 2003 | 18.3995 |
| 2002 | 18.4572 |
| 2001 | 18.5146 |
| 2000 | 18.5907 |
| 1999 | 18.7598 |
| 1998 | 18.8957 |
| 1997 | 19.0074 |
| 1996 | 19.094 |
| 1995 | 19.1057 |
| 1994 | 19.0145 |
| 1993 | 18.9112 |
| 1992 | 18.837 |
| 1991 | 18.6802 |
| 1990 | 18.5681 |
| 1989 | 18.5192 |
| 1988 | 18.2746 |
| 1987 | 17.9951 |
| 1986 | 17.8678 |
| 1985 | 17.7198 |
| 1984 | 17.5044 |
| 1983 | 17.3502 |
| 1982 | 17.2713 |
| 1981 | 17.143 |
| 1980 | 17.0246 |
| 1979 | 16.9319 |
| 1978 | 16.8085 |
| 1977 | 16.6367 |
| 1976 | 16.4162 |
| 1975 | 16.2095 |
| 1974 | 16.0571 |
| 1973 | 15.9409 |
| 1972 | 15.8619 |
| 1971 | 15.8394 |
| 1970 | 15.8339 |
| 1969 | 15.8419 |
| 1968 | 15.924 |
| 1967 | 15.922 |
| 1966 | 15.9202 |
| 1965 | 15.959 |
| 1964 | 15.8801 |
| 1963 | 15.7592 |
| 1962 | 15.6583 |
| 1961 | 15.5753 |
| 1960 | 15.3957 |