| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 28.6 | 83 |
| 2015 | 28.7 | 82 |
| 2014 | 28.8 | 83 |
| 2013 | 28.8 | 83 |
| 2012 | 28.9 | 86 |
| 2011 | 29 | 89 |
| 2010 | 29 | 92 |
| 2009 | 29.4 | 92 |
| 2008 | 29.8 | 91 |
| 2007 | 30.1 | 91 |
| 2006 | 30.4 | 93 |
| 2005 | 30.5 | 95 |
| 2004 | 31.4 | 92 |
| 2003 | 32.2 | 92 |
| 2002 | 33.1 | 92 |
| 2001 | 34 | 89 |
| 2000 | 35 | 86 |
| 1999 | 35.3 | 90 |
| 1998 | 35.7 | 90 |
| 1997 | 36.1 | 90 |
| 1996 | 36.4 | 91 |
| 1995 | 36.6 | 92 |
| 1994 | 37.1 | 91 |
| 1993 | 37.5 | 91 |
| 1992 | 37.8 | 91 |
| 1991 | 38.1 | 93 |
| 1990 | 38.4 | 93 |
| 1989 | 38.6 | 92 |
| 1988 | 38.8 | 92 |
| 1987 | 38.9 | 93 |
| 1986 | 39 | 95 |