| 年份 | 单位：% |
| 2011 | 1.38 |
| 2010 | 1.65 |
| 2009 | 1.79 |
| 2008 | 1.77 |
| 2007 | 2.22 |
| 2006 | 2.36 |
| 2005 | 2.58 |
| 2004 | 2.63 |
| 2003 | 2.86 |
| 2002 | 3.49 |
| 2001 | 3.62 |
| 2000 | 4.26 |
| 1999 | 4.93 |
| 1998 | 5.96 |
| 1997 | 6.82 |
| 1996 | 7.00 |
| 1995 | 7.48 |
| 1994 | 7.37 |
| 1993 | 7.30 |
| 1992 | 7.80 |
| 1991 | 7.32 |
| 1990 | 6.15 |
| 1989 | 4.90 |
| 1988 | 4.06 |
| 1987 | 2.84 |
| 1986 | 1.89 |
| 1985 | 2.34 |
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