| 年份 | 单位：% |
| 2011 | 3.70 |
| 2010 | 4.06 |
| 2009 | 3.92 |
| 2008 | 4.11 |
| 2007 | 4.70 |
| 2006 | 5.00 |
| 2005 | 5.37 |
| 2004 | 5.69 |
| 2003 | 5.79 |
| 2002 | 6.36 |
| 2001 | 6.18 |
| 2000 | 6.59 |
| 1999 | 7.17 |
| 1998 | 7.39 |
| 1997 | 8.11 |
| 1996 | 8.21 |
| 1995 | 8.02 |
| 1994 | 7.56 |
| 1993 | 7.67 |
| 1992 | 6.82 |
| 1991 | 5.74 |
| 1990 | 4.47 |
| 1989 | 3.51 |
| 1988 | 2.65 |
| 1987 | 1.83 |
| 1986 | 1.29 |
| 1985 | 1.58 |
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