| 年份 | 单位：% |
| 2011 | 0.33 |
| 2010 | 0.29 |
| 2009 | 0.25 |
| 2008 | 0.21 |
| 2007 | 0.24 |
| 2006 | 0.30 |
| 2005 | 0.35 |
| 2004 | 0.42 |
| 2003 | 0.49 |
| 2002 | 0.52 |
| 2001 | 0.64 |
| 2000 | 0.79 |
| 1999 | 0.84 |
| 1998 | 0.90 |
| 1997 | 0.95 |
| 1996 | 1.05 |
| 1995 | 1.06 |
| 1994 | 1.07 |
| 1993 | 1.20 |
| 1992 | 1.32 |
| 1991 | 1.57 |
| 1990 | 1.23 |
| 1989 | 1.12 |
| 1988 | 1.01 |
| 1987 | 0.73 |
| 1986 | 0.47 |
| 1985 | 0.42 |
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