| 年份 | 单位：% |
| 2010 | 6.89 |
| 2009 | 5.50 |
| 2008 | 7.40 |
| 2007 | 7.80 |
| 2006 | 7.30 |
| 2005 | 7.30 |
| 2004 | 7.50 |
| 2003 | 8.30 |
| 2002 | 9.80 |
| 2001 | 8.10 |
| 2000 | 6.50 |
| 1999 | 6.80 |
| 1998 | 5.30 |
| 1997 | 5.10 |
| 1996 | 24.30 |
| 1995 | 13.20 |
| 1994 | 15.20 |
| 1993 | 17.20 |
| 1992 | 44.30 |
| 1991 | 42.60 |
| 1990 | 39.60 |
| 1989 | 40.30 |
| 1988 | 38.40 |
| 1987 | 40.80 |
| 1986 | 41.20 |
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