| 年份 | 单位：% |
| 2010 | 12.70 |
| 2009 | 13.70 |
| 2008 | 14.30 |
| 2007 | 15.00 |
| 2006 | 14.70 |
| 2005 | 14.80 |
| 2004 | 14.10 |
| 2003 | 14.70 |
| 2002 | 16.50 |
| 2001 | 17.00 |
| 2000 | 17.30 |
| 1999 | 18.50 |
| 1998 | 18.60 |
| 1997 | 26.30 |
| 1996 | 29.30 |
| 1995 | 29.40 |
| 1994 | 30.30 |
| 1993 | 30.70 |
| 1992 | 31.30 |
| 1991 | 30.90 |
| 1990 | 30.50 |
| 1985 | 29.00 |
| 1980 | 28.20 |
| 1978 | 25.20 |
| 1975 | 24.30 |
| 1970 | 22.60 |
| 1965 | 17.70 |
| 1962 | 18.60 |
| 1957 | 14.90 |
| 1952 | 11.90 |
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