| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 17.9594 |
| 2021 | 18.243 |
| 2020 | 18.5113 |
| 2019 | 18.7359 |
| 2018 | 18.911 |
| 2017 | 19.0559 |
| 2016 | 19.1742 |
| 2015 | 19.2972 |
| 2014 | 19.4234 |
| 2013 | 19.537 |
| 2012 | 19.6422 |
| 2011 | 19.7438 |
| 2010 | 19.8669 |
| 2009 | 20.0068 |
| 2008 | 20.1357 |
| 2007 | 20.2652 |
| 2006 | 20.4195 |
| 2005 | 20.6109 |
| 2004 | 20.8156 |
| 2003 | 20.9924 |
| 2002 | 21.1399 |
| 2001 | 21.2853 |
| 2000 | 21.4254 |
| 1999 | 21.5255 |
| 1998 | 21.5897 |
| 1997 | 21.6484 |
| 1996 | 21.7203 |
| 1995 | 21.8025 |
| 1994 | 21.8609 |
| 1993 | 21.8586 |
| 1992 | 21.792 |
| 1991 | 21.6839 |
| 1990 | 21.5566 |
| 1989 | 21.4428 |
| 1988 | 21.359 |
| 1987 | 21.3187 |
| 1986 | 21.3753 |
| 1985 | 21.5317 |
| 1984 | 21.7107 |
| 1983 | 21.8836 |
| 1982 | 22.051 |
| 1981 | 22.2273 |
| 1980 | 22.4726 |
| 1979 | 22.8256 |
| 1978 | 23.2887 |
| 1977 | 23.8291 |
| 1976 | 24.4399 |
| 1975 | 25.1123 |
| 1974 | 25.784 |
| 1973 | 26.4552 |
| 1972 | 27.1327 |
| 1971 | 27.7493 |
| 1970 | 28.2838 |
| 1969 | 28.7856 |
| 1968 | 29.28 |
| 1967 | 29.7188 |
| 1966 | 30.2537 |
| 1965 | 30.5807 |
| 1964 | 30.5411 |
| 1963 | 30.5746 |
| 1962 | 30.7465 |
| 1961 | 30.9119 |
| 1960 | 30.8151 |