| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 31.4 | 71 |
| 2015 | 31.6 | 70 |
| 2014 | 31.8 | 70 |
| 2013 | 32.1 | 71 |
| 2012 | 32.5 | 69 |
| 2011 | 32.9 | 70 |
| 2010 | 33.3 | 70 |
| 2009 | 33.9 | 70 |
| 2008 | 34.6 | 70 |
| 2007 | 35.4 | 70 |
| 2006 | 36.2 | 69 |
| 2005 | 37.1 | 68 |
| 2004 | 37.8 | 67 |
| 2003 | 38.5 | 65 |
| 2002 | 39.3 | 65 |
| 2001 | 40.3 | 62 |
| 2000 | 41.6 | 50 |
| 1999 | 43 | 46 |
| 1998 | 44.4 | 33 |
| 1997 | 45.6 | 24 |
| 1996 | 46.3 | 17 |
| 1995 | 46.5 | 18 |
| 1994 | 46.6 | 18 |
| 1993 | 46.1 | 23 |
| 1992 | 45.3 | 36 |
| 1991 | 44.6 | 47 |
| 1990 | 44.3 | 50 |
| 1989 | 43.7 | 62 |
| 1988 | 43.5 | 63 |
| 1987 | 43.6 | 62 |
| 1986 | 43.4 | 63 |