| 年份 | 单位：% |
| 2011 | 2.24 |
| 2010 | 1.79 |
| 2009 | 1.71 |
| 2008 | 1.23 |
| 2007 | 1.33 |
| 2006 | 1.44 |
| 2005 | 1.44 |
| 2004 | 1.47 |
| 2003 | 1.69 |
| 2002 | 1.51 |
| 2001 | 1.57 |
| 2000 | 1.41 |
| 1999 | 1.46 |
| 1998 | 1.57 |
| 1997 | 1.52 |
| 1996 | 1.54 |
| 1995 | 1.51 |
| 1994 | 1.51 |
| 1993 | 1.43 |
| 1992 | 1.55 |
| 1991 | 1.78 |
| 1990 | 1.40 |
| 1989 | 1.12 |
| 1988 | 0.96 |
| 1987 | 0.83 |
| 1986 | 0.60 |
| 1985 | 0.58 |
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