| 年份 | 单位：% |
| 2011 | 29.55 |
| 2010 | 30.89 |
| 2009 | 30.49 |
| 2008 | 28.94 |
| 2007 | 30.10 |
| 2006 | 28.27 |
| 2005 | 28.36 |
| 2004 | 26.83 |
| 2003 | 25.43 |
| 2002 | 23.30 |
| 2001 | 20.27 |
| 2000 | 16.46 |
| 1999 | 17.22 |
| 1998 | 17.62 |
| 1997 | 18.08 |
| 1996 | 17.63 |
| 1995 | 17.15 |
| 1994 | 16.99 |
| 1993 | 16.28 |
| 1992 | 12.84 |
| 1991 | 9.79 |
| 1990 | 6.54 |
| 1989 | 5.03 |
| 1988 | 3.70 |
| 1987 | 2.28 |
| 1986 | 2.04 |
| 1985 | 3.21 |
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