| 年份 | 单位：% |
| 2011 | 26.74 |
| 2010 | 26.54 |
| 2009 | 28.00 |
| 2008 | 29.60 |
| 2007 | 30.89 |
| 2006 | 30.06 |
| 2005 | 28.36 |
| 2004 | 26.28 |
| 2003 | 25.21 |
| 2002 | 24.47 |
| 2001 | 24.12 |
| 2000 | 24.36 |
| 1999 | 22.59 |
| 1998 | 21.35 |
| 1997 | 19.24 |
| 1996 | 19.12 |
| 1995 | 17.67 |
| 1994 | 17.16 |
| 1993 | 17.27 |
| 1992 | 17.70 |
| 1991 | 16.09 |
| 1990 | 15.52 |
| 1989 | 16.04 |
| 1988 | 15.33 |
| 1987 | 14.76 |
| 1986 | 16.02 |
| 1985 | 16.62 |
| 1984 | 16.07 |
| 1983 | 16.64 |
| 1982 | 17.27 |
| 1981 | 18.08 |
| 1980 | 17.85 |
| 1979 | 17.73 |
| 1978 | 20.45 |
| 1977 | 17.36 |
| 1976 | 16.20 |
| 1975 | 16.14 |
| 1974 | 14.32 |
| 1973 | 15.08 |
| 1972 | 14.06 |
| 1971 | 12.98 |
| 1970 | 13.55 |
| 1969 | 12.88 |
| 1968 | 12.51 |
| 1967 | 12.40 |
| 1966 | 12.34 |
| 1965 | 12.41 |
| 1964 | 11.57 |
| 1963 | 10.99 |
| 1962 | 10.73 |
| 1961 | 10.19 |
| 1960 | 9.73 |
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