| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 0.8 | 46 |
| 2015 | 0.5 | 52 |
| 2014 | 0.7 | 61 |
| 2013 | 0.4 | 67 |
| 2012 | 0.3 | 75 |
| 2011 | 0.5 | 73 |
| 2010 | 0.6 | 70 |
| 2009 | 0.5 | 68 |
| 2008 | 0.9 | 70 |
| 2007 | 1.7 | 65 |
| 2006 | 1.5 | 65 |
| 2005 | 2.6 | 52 |
| 2004 | 2.8 | 52 |
| 2003 | 2.3 | 51 |
| 2002 | 1.2 | 56 |
| 2001 | 2.5 | 50 |
| 2000 | 3.2 | 50 |
| 1999 | 4.2 | 48 |
| 1998 | 8.7 | 40 |
| 1997 | 13 | 33 |
| 1996 | 11.5 | 27 |
| 1995 | 12.4 | 25 |
| 1994 | 43.8 | 7 |
| 1993 | 21 | 14 |
| 1992 | 6.9 | 22 |
| 1991 | 2.4 | 29 |
| 1990 | 0.6 | 32 |
| 1989 | 0.5 | 32 |
| 1988 | 3.4 | 27 |
| 1987 | 0.1 | 33 |
| 1986 | 0.2 | 31 |