| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 0.1 | 52 |
| 2015 | 0.2 | 53 |
| 2014 | 0.2 | 69 |
| 2013 | 0.2 | 73 |
| 2012 | 0.2 | 76 |
| 2011 | 0.2 | 78 |
| 2010 | 0.3 | 74 |
| 2009 | 0.5 | 65 |
| 2008 | 3.2 | 55 |
| 2007 | 0.5 | 72 |
| 2006 | 0.6 | 69 |
| 2005 | 0.6 | 66 |
| 2004 | 1.1 | 70 |
| 2003 | 1 | 63 |
| 2002 | 1.3 | 55 |
| 2001 | 2 | 53 |
| 2000 | 5.7 | 42 |
| 1999 | 4.3 | 47 |
| 1998 | 7.7 | 43 |
| 1997 | 5 | 47 |
| 1996 | 2.4 | 50 |
| 1995 | 1 | 55 |
| 1994 | 1.5 | 51 |
| 1993 | 1.3 | 46 |
| 1992 | 0.7 | 34 |
| 1991 | 1 | 33 |
| 1990 | 1.5 | 30 |
| 1989 | 2 | 29 |
| 1988 | 2.6 | 28 |
| 1987 | 1.5 | 30 |
| 1986 | 1.7 | 29 |