| 年份 | 单位：% |
| 2010 | 6.70 |
| 2009 | 7.40 |
| 2008 | 6.30 |
| 2007 | 7.40 |
| 2006 | 6.40 |
| 2005 | 8.70 |
| 2004 | 9.00 |
| 2003 | 7.90 |
| 2002 | 6.80 |
| 2001 | 6.70 |
| 2000 | 6.00 |
| 1999 | 5.30 |
| 1998 | 4.80 |
| 1997 | 5.40 |
| 1996 | 27.00 |
| 1995 | 15.10 |
| 1994 | 13.20 |
| 1993 | 15.10 |
| 1992 | 43.60 |
| 1991 | 40.90 |
| 1990 | 38.30 |
| 1989 | 39.00 |
| 1988 | 39.30 |
| 1987 | 40.30 |
| 1986 | 40.60 |
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