| 年度 | 实际值 |
|---|---|
| 1992 | 15.89 |
| 1993 | 16.24 |
| 1994 | 16.64 |
| 1995 | 17.11 |
| 1996 | 17.62 |
| 1997 | 17.87 |
| 1998 | 18.05 |
| 1999 | 18.14 |
| 2000 | 18.4 |
| 2001 | 18.75 |
| 2002 | 19.18 |
| 2003 | 19.39 |
| 2004 | 19.79 |
| 2005 | 19 |
| 2006 | 18.78 |
| 2007 | 19.14 |
| 2008 | 19.18 |
| 2009 | |
| 2010 | 19.21 |
| 2011 | 19 |
| 2012 | 18.9 |
| 2013 | 21 |
| 2014 | 21.5 |
| 2015 | 20 |
| 2016 | 19 |
| 2017 | 19 |
| 2018 | 18 |