| 年份 | 单位：% |
| 2009 | 6.33 |
| 2008 | 6.16 |
| 2007 | 6.03 |
| 2006 | 6.32 |
| 2005 | 6.75 |
| 2004 | 7.19 |
| 2003 | 7.59 |
| 2002 | 7.64 |
| 2001 | 7.23 |
| 2000 | 6.83 |
| 1999 | 7.58 |
| 1998 | 8.30 |
| 1997 | 9.10 |
| 1996 | 9.73 |
| 1995 | 9.54 |
| 1994 | 10.43 |
| 1993 | 11.42 |
| 1992 | 11.24 |
| 1991 | 10.32 |
| 1990 | 8.16 |
| 1989 | 7.58 |
| 1988 | 7.75 |
| 1987 | 8.81 |
| 1986 | 9.65 |
| 1985 | 10.62 |
| 1984 | 11.31 |
| 1983 | 11.93 |
| 1982 | 11.06 |
| 1981 | 7.58 |
| 1980 | 7.51 |
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