| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 11.8 | 23 |
| 2015 | 13.1 | 23 |
| 2014 | 14.8 | 29 |
| 2013 | 16.4 | 28 |
| 2012 | 14.3 | 28 |
| 2011 | 12.2 | 33 |
| 2010 | 11.1 | 38 |
| 2009 | 8.5 | 44 |
| 2008 | 7.1 | 48 |
| 2007 | 16.2 | 43 |
| 2006 | 7.5 | 48 |
| 2005 | 5.2 | 44 |
| 2004 | 3.5 | 48 |
| 2003 | 2.9 | 47 |
| 2002 | 2.6 | 44 |
| 2001 | 4 | 46 |
| 2000 | 9.1 | 36 |
| 1999 | 23.3 | 29 |
| 1998 | 14.5 | 31 |
| 1997 | 24.7 | 21 |
| 1996 | 30.8 | 12 |
| 1995 | 19.8 | 17 |
| 1994 | 21.7 | 15 |
| 1993 | 12.1 | 22 |
| 1992 | 11.8 | 16 |
| 1991 | 7.6 | 19 |
| 1990 | 6.9 | 23 |
| 1989 | 14.3 | 15 |
| 1988 | 6.5 | 16 |
| 1987 | 14.3 | 13 |
| 1986 | 5.8 | 20 |