| Year | Value | | 2017 | 2.8811202391021 | | 2016 | 2.7259420133158 | | 2015 | 2.6935678322957 | | 2014 | 2.7676371588991 | | 2013 | 2.8162376937013 | | 2012 | 2.7789557713626 | | 2011 | 2.6840540899764 | | 2010 | 2.6200836869553 | | 2009 | 2.6071042970219 | | 2008 | 2.6531679782767 | | 2007 | 2.6765044198268 | | 2006 | 2.6713890297924 | | 2005 | 2.6314898864706 | | 2003 | 2.8912105483654 | | 2002 | 2.6695184893171 | | 2001 | 2.7688524783785 | | 2000 | 2.9001038086606 | | 1991 | 3.4204697000971 | | 1990 | 3.6374357395362 | | 1985 | 4.1198348348081 | | 1980 | 3.5861761858786 | | 1975 | 2.3949498700938 | | 1970 | 3.0677412178818 |
|