| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 28.9 | 81 |
| 2015 | 29.4 | 78 |
| 2014 | 30 | 78 |
| 2013 | 30.5 | 76 |
| 2012 | 31 | 75 |
| 2011 | 31.5 | 76 |
| 2010 | 32.1 | 75 |
| 2009 | 32.6 | 75 |
| 2008 | 33.1 | 75 |
| 2007 | 33.5 | 75 |
| 2006 | 34 | 76 |
| 2005 | 34.4 | 77 |
| 2004 | 35 | 77 |
| 2003 | 35.5 | 77 |
| 2002 | 36.1 | 78 |
| 2001 | 36.6 | 77 |
| 2000 | 37.1 | 79 |
| 1999 | 37.7 | 78 |
| 1998 | 38.3 | 78 |
| 1997 | 38.8 | 79 |
| 1996 | 39.4 | 79 |
| 1995 | 39.9 | 76 |
| 1994 | 40.5 | 77 |
| 1993 | 41 | 74 |
| 1992 | 41.5 | 72 |
| 1991 | 42 | 71 |
| 1990 | 42.3 | 70 |
| 1989 | 42.7 | 67 |
| 1988 | 43.1 | 65 |
| 1987 | 43.3 | 65 |
| 1986 | 43.6 | 62 |