| 年份 | 单位：% |
| 2011 | 22.51 |
| 2010 | 23.12 |
| 2009 | 23.95 |
| 2008 | 23.09 |
| 2007 | 23.37 |
| 2006 | 22.95 |
| 2005 | 22.25 |
| 2004 | 21.99 |
| 2003 | 22.16 |
| 2002 | 21.87 |
| 2001 | 21.98 |
| 2000 | 21.47 |
| 1999 | 21.87 |
| 1998 | 21.77 |
| 1997 | 20.87 |
| 1996 | 21.94 |
| 1995 | 20.38 |
| 1994 | 21.05 |
| 1993 | 21.39 |
| 1992 | 20.80 |
| 1991 | 20.01 |
| 1990 | 19.69 |
| 1989 | 19.18 |
| 1988 | 19.82 |
| 1987 | 17.46 |
| 1986 | 17.11 |
| 1985 | 16.73 |
| 1984 | 16.47 |
| 1983 | 16.50 |
| 1982 | 16.65 |
| 1981 | 15.48 |
| 1980 | 15.46 |
| 1979 | 16.20 |
| 1978 | 17.11 |
| 1977 | 16.99 |
| 1976 | 15.58 |
| 1975 | 13.89 |
| 1974 | 12.14 |
| 1973 | 14.07 |
| 1972 | 14.36 |
| 1971 | 13.44 |
| 1970 | 13.43 |
| 1969 | 12.78 |
| 1968 | 12.01 |
| 1967 | 11.73 |
| 1966 | 11.64 |
| 1965 | 13.45 |
| 1964 | 12.97 |
| 1963 | 12.14 |
| 1962 | 10.84 |
| 1961 | 10.87 |
| 1960 | 9.72 |
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