| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 17.1281 |
| 2021 | 16.6789 |
| 2020 | 16.2234 |
| 2019 | 15.7918 |
| 2018 | 15.3977 |
| 2017 | 15.0663 |
| 2016 | 14.6723 |
| 2015 | 14.3278 |
| 2014 | 14.1356 |
| 2013 | 13.8911 |
| 2012 | 13.5844 |
| 2011 | 13.2702 |
| 2010 | 13.0282 |
| 2009 | 12.8563 |
| 2008 | 12.6709 |
| 2007 | 12.5078 |
| 2006 | 12.4221 |
| 2005 | 12.3602 |
| 2004 | 12.3047 |
| 2003 | 12.2779 |
| 2002 | 12.2875 |
| 2001 | 12.2969 |
| 2000 | 12.3176 |
| 1999 | 12.3962 |
| 1998 | 12.4615 |
| 1997 | 12.514 |
| 1996 | 12.5504 |
| 1995 | 12.5436 |
| 1994 | 12.484 |
| 1993 | 12.4273 |
| 1992 | 12.3968 |
| 1991 | 12.3269 |
| 1990 | 12.2845 |
| 1989 | 12.2749 |
| 1988 | 12.1508 |
| 1987 | 11.9995 |
| 1986 | 11.9189 |
| 1985 | 11.8115 |
| 1984 | 11.6626 |
| 1983 | 11.5495 |
| 1982 | 11.4801 |
| 1981 | 11.3815 |
| 1980 | 11.2786 |
| 1979 | 11.175 |
| 1978 | 11.0386 |
| 1977 | 10.8648 |
| 1976 | 10.655 |
| 1975 | 10.4457 |
| 1974 | 10.2682 |
| 1973 | 10.1118 |
| 1972 | 9.97579 |
| 1971 | 9.87926 |
| 1970 | 9.80325 |
| 1969 | 9.73887 |
| 1968 | 9.71449 |
| 1967 | 9.65321 |
| 1966 | 9.57878 |
| 1965 | 9.55391 |
| 1964 | 9.51858 |
| 1963 | 9.45139 |
| 1962 | 9.37584 |
| 1961 | 9.31051 |
| 1960 | 9.23044 |