| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | - |
| 2021 | 6.8 |
| 2020 | 6.9 |
| 2019 | 7 |
| 2018 | 7.2 |
| 2017 | 7.2 |
| 2016 | 7.3 |
| 2015 | 7.4 |
| 2014 | 7.5 |
| 2013 | 7.6 |
| 2012 | 7.7 |
| 2011 | 7.9 |
| 2010 | 8 |
| 2009 | 8.2 |
| 2008 | 8.3 |
| 2007 | 8.5 |
| 2006 | 8.6 |
| 2005 | 8.8 |
| 2004 | 8.9 |
| 2003 | 9 |
| 2002 | 9.1 |
| 2001 | 9.1 |
| 2000 | 9.3 |
| 1999 | 9.4 |
| 1998 | 9.6 |
| 1997 | 9.8 |
| 1996 | 10.1 |
| 1995 | 10.4 |
| 1994 | 10.8 |
| 1993 | 11.2 |
| 1992 | 11.6 |
| 1991 | 12.1 |
| 1990 | 12.5 |
| 1989 | 12.8 |
| 1988 | 13.1 |
| 1987 | 13.5 |
| 1986 | 13.8 |
| 1985 | 14.2 |
| 1984 | 14.4 |
| 1983 | 14.8 |
| 1982 | 15.4 |
| 1981 | 15.9 |
| 1980 | 16.6 |
| 1979 | 17.4 |
| 1978 | 18.1 |
| 1977 | 19 |
| 1976 | 19.9 |
| 1975 | 20.9 |
| 1974 | 22 |
| 1973 | 23 |
| 1972 | 24.1 |
| 1971 | 25 |
| 1970 | 26 |
| 1969 | 27 |
| 1968 | 27.9 |
| 1967 | 28.7 |
| 1966 | 29.5 |
| 1965 | 30.3 |
| 1964 | 31 |
| 1963 | 31.8 |
| 1962 | 32.4 |
| 1961 | 33 |
| 1960 | 33.7 |