| 年份 | 单位：% |
| 2011 | 22.84 |
| 2010 | 22.97 |
| 2009 | 22.88 |
| 2008 | 21.24 |
| 2007 | 21.95 |
| 2006 | 20.65 |
| 2005 | 20.04 |
| 2004 | 18.72 |
| 2003 | 17.70 |
| 2002 | 15.89 |
| 2001 | 13.45 |
| 2000 | 10.67 |
| 1999 | 11.00 |
| 1998 | 11.00 |
| 1997 | 11.03 |
| 1996 | 10.79 |
| 1995 | 10.36 |
| 1994 | 9.93 |
| 1993 | 9.19 |
| 1992 | 7.31 |
| 1991 | 5.57 |
| 1990 | 4.23 |
| 1989 | 3.31 |
| 1988 | 2.47 |
| 1987 | 1.71 |
| 1986 | 1.49 |
| 1985 | 2.17 |
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