| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | - |
| 2021 | 79.3 |
| 2020 | 79.9 |
| 2019 | 81.4 |
| 2018 | 81.2 |
| 2017 | 81.1 |
| 2016 | 81.1 |
| 2015 | 81.2 |
| 2014 | 81.3 |
| 2013 | 81.2 |
| 2012 | 81.2 |
| 2011 | 81.1 |
| 2010 | 81 |
| 2009 | 80.9 |
| 2008 | 80.6 |
| 2007 | 80.6 |
| 2006 | 80.3 |
| 2005 | 80.1 |
| 2004 | 80.1 |
| 2003 | 79.7 |
| 2002 | 79.6 |
| 2001 | 79.5 |
| 2000 | 79.3 |
| 1999 | 79.4 |
| 1998 | 79.5 |
| 1997 | 79.4 |
| 1996 | 79.1 |
| 1995 | 78.9 |
| 1994 | 79 |
| 1993 | 78.8 |
| 1992 | 79.1 |
| 1991 | 78.9 |
| 1990 | 78.8 |
| 1989 | 78.5 |
| 1988 | 78.3 |
| 1987 | 78.3 |
| 1986 | 78.2 |
| 1985 | 78.2 |
| 1984 | 78.2 |
| 1983 | 78.1 |
| 1982 | 78.1 |
| 1981 | 77.8 |
| 1980 | 77.4 |
| 1979 | 77.8 |
| 1978 | 77.3 |
| 1977 | 77.2 |
| 1976 | 76.8 |
| 1975 | 76.6 |
| 1974 | 75.9 |
| 1973 | 75.3 |
| 1972 | 75.1 |
| 1971 | 75 |
| 1970 | 74.7 |
| 1969 | 74.4 |
| 1968 | 74.1 |
| 1967 | 74.3 |
| 1966 | 73.9 |
| 1965 | 73.8 |
| 1964 | 73.7 |
| 1963 | 73.4 |
| 1962 | 73.5 |
| 1961 | 73.6 |
| 1960 | 73.1 |