| 年份 | 单位：% |
| 2011 | 24.56 |
| 2010 | 25.92 |
| 2009 | 25.26 |
| 2008 | 25.15 |
| 2007 | 24.23 |
| 2006 | 24.90 |
| 2005 | 23.07 |
| 2004 | 24.67 |
| 2003 | 24.68 |
| 2002 | 22.70 |
| 2001 | 22.94 |
| 2000 | 22.77 |
| 1999 | 21.21 |
| 1998 | 21.73 |
| 1997 | 21.56 |
| 1996 | 21.85 |
| 1995 | 20.38 |
| 1994 | 21.73 |
| 1993 | 21.23 |
| 1992 | 19.87 |
| 1991 | 19.33 |
| 1990 | 18.14 |
| 1989 | 17.99 |
| 1988 | 20.45 |
| 1987 | 19.33 |
| 1986 | 18.18 |
| 1985 | 18.73 |
| 1984 | 18.32 |
| 1983 | 19.05 |
| 1982 | 19.35 |
| 1981 | 20.24 |
| 1980 | 19.12 |
| 1979 | 18.11 |
| 1978 | 17.86 |
| 1977 | 19.02 |
| 1976 | 17.74 |
| 1975 | 17.77 |
| 1974 | 16.45 |
| 1973 | 16.68 |
| 1972 | 15.55 |
| 1971 | 14.48 |
| 1970 | 14.02 |
| 1969 | 16.07 |
| 1968 | 16.82 |
| 1967 | 15.54 |
| 1966 | 15.02 |
| 1965 | 12.02 |
| 1964 | 11.90 |
| 1963 | 12.62 |
| 1962 | 12.80 |
| 1961 | 9.28 |
| 1960 | 9.21 |
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