| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 8.25 | 77 |
| 2015 | 7.96 | 77 |
| 2014 | 7.68 | 78 |
| 2013 | 7.42 | 80 |
| 2012 | 7.18 | 80 |
| 2011 | 6.95 | 81 |
| 2010 | 6.73 | 83 |
| 2009 | 6.55 | 86 |
| 2008 | 6.38 | 86 |
| 2007 | 6.22 | 87 |
| 2006 | 6.07 | 87 |
| 2005 | 5.91 | 89 |
| 2004 | 5.74 | 90 |
| 2003 | 5.57 | 91 |
| 2002 | 5.39 | 92 |
| 2001 | 5.22 | 93 |
| 2000 | 5.07 | 94 |
| 1999 | 4.94 | 96 |
| 1998 | 4.82 | 97 |
| 1997 | 4.72 | 98 |
| 1996 | 4.61 | 100 |
| 1995 | 4.49 | 101 |
| 1994 | 4.42 | 100 |
| 1993 | 4.33 | 100 |
| 1992 | 4.24 | 103 |
| 1991 | 4.14 | 102 |
| 1990 | 4.04 | 104 |
| 1989 | 3.99 | 105 |
| 1988 | 3.94 | 105 |
| 1987 | 3.9 | 107 |
| 1986 | 3.86 | 110 |