| 时间 | 值 （%） | 世界排名 |
|---|---|---|
| 2016 | 6.4 | 62 |
| 2015 | 6.25 | 58 |
| 2014 | 6.23 | 59 |
| 2013 | 6.3 | 62 |
| 2012 | 6.45 | 67 |
| 2011 | 6.62 | 69 |
| 2010 | 6.8 | 70 |
| 2009 | 7.01 | 74 |
| 2008 | 7.22 | 78 |
| 2007 | 7.42 | 82 |
| 2006 | 7.6 | 85 |
| 2005 | 7.74 | 89 |
| 2004 | 7.86 | 91 |
| 2003 | 7.93 | 92 |
| 2002 | 7.95 | 92 |
| 2001 | 7.95 | 92 |
| 2000 | 7.95 | 93 |
| 1999 | 7.99 | 95 |
| 1998 | 8.04 | 97 |
| 1997 | 8.09 | 101 |
| 1996 | 8.1 | 102 |
| 1995 | 8.04 | 103 |
| 1994 | 7.99 | 104 |
| 1993 | 7.87 | 103 |
| 1992 | 7.71 | 102 |
| 1991 | 7.56 | 96 |
| 1990 | 7.45 | 94 |
| 1989 | 7.49 | 96 |
| 1988 | 7.59 | 103 |
| 1987 | 7.75 | 108 |
| 1986 | 7.93 | 111 |