| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | - |
| 2021 | 3.3 |
| 2020 | 3.4 |
| 2019 | 3.5 |
| 2018 | 3.7 |
| 2017 | 3.8 |
| 2016 | 3.8 |
| 2015 | 3.9 |
| 2014 | 4 |
| 2013 | 4 |
| 2012 | 4.1 |
| 2011 | 4.1 |
| 2010 | 4.1 |
| 2009 | 4.2 |
| 2008 | 4.3 |
| 2007 | 4.3 |
| 2006 | 4.4 |
| 2005 | 4.5 |
| 2004 | 4.6 |
| 2003 | 4.6 |
| 2002 | 4.6 |
| 2001 | 4.6 |
| 2000 | 4.6 |
| 1999 | 4.7 |
| 1998 | 4.8 |
| 1997 | 4.8 |
| 1996 | 4.9 |
| 1995 | 5 |
| 1994 | 5.1 |
| 1993 | 5.3 |
| 1992 | 5.4 |
| 1991 | 5.6 |
| 1990 | 5.8 |
| 1989 | 6 |
| 1988 | 6.3 |
| 1987 | 6.5 |
| 1986 | 6.7 |
| 1985 | 7 |
| 1984 | 7.2 |
| 1983 | 7.5 |
| 1982 | 7.7 |
| 1981 | 8 |
| 1980 | 8.4 |
| 1979 | 8.9 |
| 1978 | 9.5 |
| 1977 | 10.1 |
| 1976 | 10.8 |
| 1975 | 11.5 |
| 1974 | 12.2 |
| 1973 | 12.9 |
| 1972 | 13.6 |
| 1971 | 14.4 |
| 1970 | 15 |
| 1969 | 15.5 |
| 1968 | 15.8 |
| 1967 | - |
| 1966 | - |
| 1965 | - |
| 1964 | - |
| 1963 | - |
| 1962 | - |
| 1961 | - |
| 1960 | - |