| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 28.7 | 82 |
| 2015 | 29 | 80 |
| 2014 | 29.3 | 80 |
| 2013 | 29.7 | 80 |
| 2012 | 30 | 79 |
| 2011 | 30.4 | 80 |
| 2010 | 30.7 | 80 |
| 2009 | 31.1 | 81 |
| 2008 | 31.5 | 81 |
| 2007 | 31.9 | 82 |
| 2006 | 32.3 | 82 |
| 2005 | 32.7 | 82 |
| 2004 | 33.1 | 86 |
| 2003 | 33.5 | 86 |
| 2002 | 34 | 86 |
| 2001 | 34.4 | 86 |
| 2000 | 34.7 | 90 |
| 1999 | 35.1 | 92 |
| 1998 | 35.5 | 92 |
| 1997 | 35.8 | 92 |
| 1996 | 36.2 | 93 |
| 1995 | 36.5 | 93 |
| 1994 | 36.8 | 94 |
| 1993 | 37.2 | 96 |
| 1992 | 37.6 | 95 |
| 1991 | 37.9 | 94 |
| 1990 | 38.2 | 94 |
| 1989 | 38.6 | 93 |
| 1988 | 38.9 | 91 |
| 1987 | 39.3 | 91 |
| 1986 | 39.7 | 90 |