| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 16.916 |
| 2021 | 17.127 |
| 2020 | 17.336 |
| 2019 | 17.541 |
| 2018 | 17.744 |
| 2017 | 17.942 |
| 2016 | 18.138 |
| 2015 | 18.329 |
| 2014 | 18.517 |
| 2013 | 18.701 |
| 2012 | 18.881 |
| 2011 | 19.056 |
| 2010 | 19.228 |
| 2009 | 19.394 |
| 2008 | 19.562 |
| 2007 | 19.731 |
| 2006 | 19.901 |
| 2005 | 20.072 |
| 2004 | 20.243 |
| 2003 | 20.417 |
| 2002 | 20.591 |
| 2001 | 20.766 |
| 2000 | 20.943 |
| 1999 | 21.258 |
| 1998 | 21.623 |
| 1997 | 21.992 |
| 1996 | 22.364 |
| 1995 | 22.743 |
| 1994 | 23.125 |
| 1993 | 23.512 |
| 1992 | 23.903 |
| 1991 | 24.299 |
| 1990 | 24.7 |
| 1989 | 24.911 |
| 1988 | 25.058 |
| 1987 | 25.207 |
| 1986 | 25.356 |
| 1985 | 25.506 |
| 1984 | 25.656 |
| 1983 | 25.806 |
| 1982 | 25.958 |
| 1981 | 26.11 |
| 1980 | 26.262 |
| 1979 | 26.308 |
| 1978 | 26.318 |
| 1977 | 26.327 |
| 1976 | 26.337 |
| 1975 | 26.347 |
| 1974 | 26.357 |
| 1973 | 26.367 |
| 1972 | 26.377 |
| 1971 | 26.387 |
| 1970 | 26.398 |
| 1969 | 26.667 |
| 1968 | 27.026 |
| 1967 | 27.388 |
| 1966 | 27.753 |
| 1965 | 28.121 |
| 1964 | 28.492 |
| 1963 | 28.866 |
| 1962 | 29.243 |
| 1961 | 29.623 |
| 1960 | 30.004 |