| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 9.7 | 27 |
| 2015 | 11 | 24 |
| 2014 | 10.6 | 33 |
| 2013 | 13.9 | 30 |
| 2012 | 12 | 31 |
| 2011 | 13.8 | 30 |
| 2010 | 14.5 | 34 |
| 2009 | 12.9 | 39 |
| 2008 | 9.7 | 42 |
| 2007 | 21 | 38 |
| 2006 | 15.4 | 38 |
| 2005 | 8.6 | 41 |
| 2004 | 7 | 43 |
| 2003 | 3.6 | 45 |
| 2002 | 1.6 | 52 |
| 2001 | 5.3 | 41 |
| 2000 | 11.9 | 34 |
| 1999 | 6.3 | 41 |
| 1998 | 5.1 | 49 |
| 1997 | 4.7 | 48 |
| 1996 | 3.5 | 45 |
| 1995 | 1.9 | 47 |
| 1994 | 4.7 | 38 |
| 1993 | 2.1 | 42 |
| 1992 | 0.4 | 35 |
| 1991 | 0.1 | 35 |