| 年份 | 单位：% |
| 2011 | 22.23 |
| 2010 | 23.50 |
| 2009 | 24.23 |
| 2008 | 21.45 |
| 2007 | 22.12 |
| 2006 | 21.13 |
| 2005 | 20.50 |
| 2004 | 20.32 |
| 2003 | 20.58 |
| 2002 | 19.97 |
| 2001 | 19.49 |
| 2000 | 19.69 |
| 1999 | 20.48 |
| 1998 | 20.21 |
| 1997 | 20.26 |
| 1996 | 20.59 |
| 1995 | 20.27 |
| 1994 | 19.81 |
| 1993 | 20.08 |
| 1992 | 19.37 |
| 1991 | 19.37 |
| 1990 | 18.25 |
| 1989 | 17.92 |
| 1988 | 18.67 |
| 1987 | 18.96 |
| 1986 | 19.46 |
| 1985 | 17.46 |
| 1984 | 17.73 |
| 1983 | 17.23 |
| 1982 | 16.45 |
| 1981 | 16.13 |
| 1980 | 15.32 |
| 1979 | 14.62 |
| 1978 | 15.29 |
| 1977 | 14.15 |
| 1976 | 13.47 |
| 1975 | 11.14 |
| 1974 | 11.03 |
| 1973 | 12.38 |
| 1972 | 14.12 |
| 1971 | 13.92 |
| 1970 | 14.36 |
| 1969 | 16.25 |
| 1968 | 16.51 |
| 1967 | 16.35 |
| 1966 | 15.87 |
| 1965 | 16.10 |
| 1964 | 15.66 |
| 1963 | 21.26 |
| 1962 | 21.06 |
| 1961 | 21.97 |
| 1960 | 20.30 |
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