| Year | Value | | 2022 | 1.6451213939414 | | 2021 | 1.7838547803357 | | 2020 | 1.8608190625872 | | 2019 | 1.8902392432778 | | 2018 | 2.096037715424 | | 2017 | 1.9244399325853 | | 2016 | 2.0738091082626 | | 2015 | 1.7496140345071 | | 2014 | 1.8228767739705 | | 2013 | 1.9519101532091 | | 2012 | 1.9145355472143 | | 2011 | 2.2186636938669 | | 2010 | 2.3742940597172 | | 2009 | 2.0987005681964 | | 2008 | 2.0061232551566 | | 2007 | 1.6443905880886 | | 2006 | 1.8099327448363 | | 2005 | 1.9028865792151 | | 2004 | 1.541330909162 | | 2003 | 1.470954078719 | | 2002 | 1.3037865159185 | | 2001 | 1.290036453126 | | 2000 | 1.4352057909751 | | 1999 | 1.6845059133606 | | 1998 | 1.6472089506266 | | 1997 | 1.4948391704749 | | 1996 | 1.6525777310162 | | 1995 | 1.6933392617787 | | 1994 | 1.818089820086 | | 1993 | 1.7480467649015 | | 1992 | 1.6327115429226 | | 1991 | 1.3845165938386 | | 1990 | 1.2759745775935 | | 1989 | 1.2528723055039 | | 1988 | 1.2600902561495 | | 1987 | 1.4292920800404 | | 1986 | 1.6345123258306 | | 1985 | 1.8411214953271 | | 1984 | 2.0308017514721 | | 1983 | 2.2558661145618 | | 1982 | 2.1995610023779 | | 1981 | 2.1661010181673 | | 1980 | 2.4235388879676 | | 1979 | 2.4584169773444 | | 1978 | 2.4330962059621 | | 1977 | 1.782319391635 |
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