| Year | Value | | 2022 | 3.2641098533933 | | 2021 | 3.3418637564212 | | 2020 | 3.1518746633491 | | 2019 | 3.6513298315727 | | 2018 | 2.5978296099921 | | 2017 | 1.7070199995213 | | 2016 | 1.838836427186 | | 2015 | 1.8633848518049 | | 2014 | 1.9467626475797 | | 2013 | 2.0086563415418 | | 2012 | 2.111057915815 | | 2011 | 2.1431442401058 | | 2010 | 2.2195635088643 | | 2009 | 2.0170824097148 | | 2008 | 1.9267036804046 | | 2007 | 1.7619798012114 | | 2006 | 1.7078492976424 | | 2005 | 1.8189745896827 | | 2004 | 1.9981694310432 | | 2003 | 1.9945437983343 | | 2002 | 1.8487023756076 | | 2001 | 1.6445990787663 | | 2000 | 1.5041778539895 | | 1999 | 1.5616795677434 | | 1998 | 1.6475304940802 | | 1997 | 1.7771075430971 | | 1996 | 2.0452392102197 | | 1995 | 2.3172650550708 | | 1994 | 2.6170300713031 | | 1993 | 2.6783819347171 | | 1992 | 2.6850998463902 | | 1991 | 2.5222645309501 | | 1990 | 2.6936512514822 | | 1989 | 2.7042577675489 | | 1988 | 2.94133549608 | | 1987 | 2.8480595948005 | | 1986 | 2.8833551769332 | | 1985 | 2.7032425594247 | | 1984 | 2.7936910362985 | | 1983 | 2.3187789182577 | | 1982 | 2.2244969291457 | | 1981 | 2.0273291925466 | | 1980 | 2.1422598967794 | | 1979 | 2.438328365781 | | 1978 | 2.5126183546402 | | 1977 | 2.2242434264925 | | 1976 | 3.0640115210806 | | 1975 | 2.2762243327851 | | 1974 | 1.8860704132954 | | 1973 | 1.9957277998169 | | 1972 | 2.2289833983729 |
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