| Year | Value | | 2022 | 8.6807215073403 | | 2021 | 4.1862150652357 | | 2020 | 3.476173727222 | | 2019 | 2.7264303738882 | | 2018 | 3.0174818103141 | | 2017 | 3.7388838849067 | | 2016 | 3.2052243455975 | | 2015 | 2.9170332470041 | | 2014 | 3.5169555136923 | | 2013 | 4.8851638759547 | | 2012 | 5.7138443419105 | | 2011 | 6.3810958246069 | | 2010 | 3.9864740829269 | | 2009 | 4.361122191198 | | 2008 | 10.687732015355 | | 2007 | 6.6760006328411 | | 2006 | 6.3894386172121 | | 2005 | 6.8809378489152 | | 2004 | 4.3193367743479 | | 2003 | 5.3023877321994 | | 2002 | 3.5330421430808 | | 2001 | 4.390650799442 | | 2000 | 3.8226008808631 | | 1999 | 4.1426542768311 | | 1998 | 5.9765085214186 | | 1997 | 6.5054871826909 | | 1996 | 7.8306309172108 | | 1995 | 9.8256965282431 | | 1994 | 15.252407292314 | | 1993 | 6.9845992092048 | | 1992 | 9.5090414619015 | | 1991 | 9.5509074131036 | | 1990 | 8.2155358474709 | | 1989 | 7.7826751173138 | | 1988 | 8.6340491918516 | | 1987 | 6.9825296521968 | | 1986 | 6.1847785445693 | | 1985 | 6.9060578214047 | | 1984 | 8.3644241182997 | | 1983 | 9.3554650798615 | | 1982 | 10.837870380065 | | 1981 | 12.167394348794 |
|