| Year | Value | | 2021 | 4.0041656505588 | | 2020 | 3.7130037287788 | | 2019 | 3.9527299713024 | | 2018 | 3.8354720538865 | | 2017 | 3.952756587923 | | 2016 | 4.3455440676847 | | 2015 | 4.5150189531603 | | 2014 | 5.0332413913405 | | 2013 | 4.9971817257389 | | 2012 | 6.0441682948377 | | 2011 | 5.3573132030421 | | 2010 | 5.7421517325713 | | 2009 | 5.5786673161711 | | 2008 | 6.428284451587 | | 2007 | 6.3618948325857 | | 2006 | 7.1366688979511 | | 2005 | 8.7809877533395 | | 2004 | 10.003830524165 | | 2003 | 12.005507957384 | | 2002 | 12.255241215503 | | 2001 | 12.991679254879 | | 2000 | 12.905059606595 | | 1999 | 10.751395838159 | | 1998 | 10.191775496793 | | 1997 | 13.658916608886 | | 1996 | 16.269238254692 | | 1995 | 18.272030457951 | | 1994 | 19.234208573443 | | 1993 | 20.392841223853 | | 1992 | 18.63725956637 | | 1991 | 20.710297759643 | | 1990 | 23.899788449506 | | 1989 | 23.354646942937 | | 1988 | 23.560764821005 | | 1987 | 20.585782075993 | | 1986 | 19.637129434635 | | 1985 | 21.169750781401 | | 1984 | 22.007389604333 | | 1983 | 19.620807224475 | | 1982 | 18.637137028925 | | 1981 | 21.348062416441 | | 1980 | 21.790263118604 | | 1979 | 23.865295912318 | | 1978 | 24.912613514605 | | 1977 | 25.24230391529 | | 1976 | 24.326928223519 | | 1975 | 24.538305204559 | | 1974 | 24.523253828201 | | 1973 | 28.258464262005 | | 1972 | 30.088919652841 |
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