| 年份 | 单位：% |
| 2011 | 5.16 |
| 2010 | 5.23 |
| 2009 | 5.56 |
| 2008 | 4.63 |
| 2007 | 5.18 |
| 2006 | 5.07 |
| 2005 | 5.17 |
| 2004 | 4.91 |
| 2003 | 5.01 |
| 2002 | 4.84 |
| 2001 | 4.48 |
| 2000 | 3.95 |
| 1999 | 4.17 |
| 1998 | 4.60 |
| 1997 | 4.79 |
| 1996 | 5.12 |
| 1995 | 5.27 |
| 1994 | 5.45 |
| 1993 | 5.32 |
| 1992 | 4.79 |
| 1991 | 4.15 |
| 1990 | 3.32 |
| 1989 | 2.94 |
| 1988 | 2.47 |
| 1987 | 1.71 |
| 1986 | 1.49 |
| 1985 | 2.17 |
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