| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 6.9 | 31 |
| 2015 | 8.4 | 31 |
| 2014 | 6.6 | 41 |
| 2013 | 6.2 | 41 |
| 2012 | 5.8 | 41 |
| 2011 | 9 | 36 |
| 2010 | 12.5 | 37 |
| 2009 | 13 | 37 |
| 2008 | 23 | 33 |
| 2007 | 33.4 | 34 |
| 2006 | 24.8 | 34 |
| 2005 | 13.8 | 37 |
| 2004 | 8.6 | 39 |
| 2003 | 4.6 | 41 |
| 2002 | 3.1 | 43 |
| 2001 | 3.7 | 47 |
| 2000 | 4.9 | 44 |
| 1999 | 5.5 | 44 |
| 1998 | 8.3 | 41 |
| 1997 | 5.7 | 46 |
| 1996 | 4.4 | 42 |
| 1995 | 5.4 | 37 |
| 1994 | 4.3 | 39 |
| 1993 | 3.5 | 39 |
| 1992 | 2.4 | 29 |
| 1991 | 4.4 | 27 |
| 1990 | 34.7 | 5 |
| 1989 | 19.7 | 9 |
| 1988 | 1.5 | 31 |
| 1987 | 1.1 | 31 |
| 1986 | 1.2 | 30 |