| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 29.2 | 79 |
| 2015 | 29.3 | 79 |
| 2014 | 29.6 | 79 |
| 2013 | 29.8 | 79 |
| 2012 | 30 | 81 |
| 2011 | 30.2 | 81 |
| 2010 | 30.4 | 82 |
| 2009 | 30.6 | 82 |
| 2008 | 30.9 | 83 |
| 2007 | 31.1 | 85 |
| 2006 | 31.4 | 86 |
| 2005 | 31.7 | 89 |
| 2004 | 32 | 90 |
| 2003 | 32.4 | 91 |
| 2002 | 32.8 | 93 |
| 2001 | 33.2 | 94 |
| 2000 | 33.6 | 97 |
| 1999 | 34.1 | 98 |
| 1998 | 34.7 | 98 |
| 1997 | 35.2 | 98 |
| 1996 | 35.7 | 98 |
| 1995 | 36.2 | 97 |
| 1994 | 36.9 | 93 |
| 1993 | 37.5 | 92 |
| 1992 | 38.1 | 89 |
| 1991 | 38.6 | 89 |
| 1990 | 39.2 | 87 |
| 1989 | 39.5 | 86 |
| 1988 | 39.9 | 85 |
| 1987 | 40.1 | 85 |
| 1986 | 40.4 | 86 |