| 年份 | 单位：% |
| 2011 | 2.30 |
| 2010 | 2.28 |
| 2009 | 2.03 |
| 2008 | 2.20 |
| 2007 | 2.30 |
| 2006 | 2.56 |
| 2005 | 2.64 |
| 2004 | 2.83 |
| 2003 | 2.51 |
| 2002 | 2.45 |
| 2001 | 2.22 |
| 2000 | 2.50 |
| 1999 | 2.32 |
| 1998 | 2.24 |
| 1997 | 2.14 |
| 1996 | 2.03 |
| 1995 | 2.08 |
| 1994 | 1.54 |
| 1993 | 1.11 |
| 1992 | 0.88 |
| 1991 | 0.41 |
| 1990 | 1.23 |
| 1989 | 1.12 |
| 1988 | 1.01 |
| 1987 | 0.73 |
| 1986 | 0.47 |
| 1985 | 0.42 |
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