| Date | Value | Change, % |
| 2015 | 55.6 | 2.21% |
| 2014 | 54.4 | 2.34% |
| 2013 | 53.2 | 2.46% |
| 2012 | 51.9 | 2.60% |
| 2011 | 50.6 | 2.74% |
| 2010 | 49.2 | 2.81% |
| 2009 | 47.9 | 2.88% |
| 2008 | 46.5 | 2.96% |
| 2007 | 45.2 | 3.03% |
| 2006 | 43.9 | 3.17% |
| 2005 | 42.5 | 3.35% |
| 2004 | 41.1 |