| Year | Value | | 2022 | 2.1528393036299 | | 2021 | 2.2025892456714 | | 2020 | 2.2764533566864 | | 2019 | 2.3362904699066 | | 2018 | 2.3723371476076 | | 2017 | 2.4378909357331 | | 2016 | 2.5188783199054 | | 2015 | 2.5826495434354 | | 2014 | 2.6479419321498 | | 2013 | 2.6885011744737 | | 2012 | 2.6797166786976 | | 2011 | 2.6320084343274 | | 2010 | 2.5869974926204 | | 2009 | 2.5710829245713 | | 2008 | 2.5764583604679 | | 2007 | 2.5471039563999 | | 2006 | 2.4611415313588 | | 2005 | 2.3988927605268 | | 2004 | 2.3553458597913 | | 2003 | 2.304691466204 | | 2002 | 2.2256014657547 | | 2001 | 2.1326015824877 | | 2000 | 2.0002119265096 | | 1999 | 1.8743555032781 | | 1998 | 1.8434675860939 | | 1997 | 1.8965878358858 | | 1996 | 2.0712699677972 | | 1995 | 2.5018988635159 | | 1994 | 3.0617688662898 | | 1993 | 3.2562928760445 | | 1992 | 2.8808592959271 | | 1991 | 2.0642115217012 | | 1990 | 1.5852986794001 | | 1989 | 1.5946110269628 | | 1988 | 1.5679823327486 | | 1987 | 1.5507249579751 | | 1986 | 1.5471505436693 | | 1985 | 1.5538593485923 | | 1984 | 1.5712786111499 | | 1983 | 1.5903060943701 | | 1982 | 1.6052357292637 | | 1981 | 1.6100141440903 | | 1980 | 1.1934372710691 | | 1979 | 3.3754750154473 | | 1978 | 6.1982980694661 | | 1977 | 6.2982782177174 | | 1976 | 6.0844059570712 | | 1975 | 5.621600150588 | | 1974 | 3.321686007772 | | 1973 | 1.2097019227462 | | 1972 | 0.24343727135095 | | 1971 | 0.4785182176103 | | 1970 | 0.64143706006216 | | 1969 | 0.75212395311989 | | 1968 | 1.745293273148 | | 1967 | 0.5254633928238 | | 1966 | -1.2653335640702 | | 1965 | -2.1990084109556 | | 1964 | 0.89709381068752 | | 1963 | 0.91098175868085 | | 1962 | 1.2427876559917 | | 1961 | 0.89127579595034 |
|