| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 27.9 | 88 |
| 2015 | 28.2 | 87 |
| 2014 | 28.5 | 85 |
| 2013 | 28.8 | 84 |
| 2012 | 29.1 | 85 |
| 2011 | 29.4 | 85 |
| 2010 | 29.8 | 87 |
| 2009 | 30.1 | 86 |
| 2008 | 30.5 | 85 |
| 2007 | 30.9 | 87 |
| 2006 | 31.3 | 87 |
| 2005 | 31.7 | 87 |
| 2004 | 32.2 | 89 |
| 2003 | 32.7 | 89 |
| 2002 | 33.2 | 90 |
| 2001 | 33.7 | 93 |
| 2000 | 34.2 | 95 |
| 1999 | 34.6 | 95 |
| 1998 | 35.1 | 95 |
| 1997 | 35.5 | 96 |
| 1996 | 35.9 | 96 |
| 1995 | 36.3 | 95 |
| 1994 | 36.7 | 97 |
| 1993 | 37 | 97 |
| 1992 | 37.4 | 97 |
| 1991 | 37.7 | 96 |
| 1990 | 37.9 | 97 |
| 1989 | 38.1 | 96 |
| 1988 | 38.3 | 97 |
| 1987 | 38.4 | 100 |
| 1986 | 38.6 | 101 |