| Year | Value | | 2022 | 4.84048780209 | | 2021 | 4.8112319150758 | | 2020 | 4.7048747958054 | | 2019 | 4.5662481883701 | | 2018 | 4.5600908711715 | | 2017 | 4.6843537414966 | | 2016 | 4.435506300705 | | 2015 | 4.1851629472753 | | 2014 | 4.2034526190789 | | 2013 | 4.1937398195659 | | 2012 | 4.6557209066212 | | 2011 | 5.4016416306769 | | 2010 | 5.741499827718 | | 2009 | 6.3913838803715 | | 2008 | 5.9949715614973 | | 2007 | 5.8512825662422 | | 2006 | 4.6593240085487 | | 2005 | 4.7953227898843 | | 2004 | 5.1417212887404 | | 2003 | 6.0010594817394 | | 2002 | 5.4460144666118 | | 2001 | 5.8930785351479 | | 2000 | 6.2565564256483 | | 1999 | 6.2822332214184 | | 1998 | 6.2746216510098 | | 1997 | 6.1315062093666 | | 1996 | 6.0269175167471 | | 1995 | 4.4542485046451 | | 1994 | 6.5394570185583 | | 1993 | 6.977109417038 | | 1992 | 6.924248719014 | | 1991 | 9.973089561725 | | 1990 | 7.7510675827897 | | 1989 | 9.0707809530092 | | 1988 | 9.3636146957676 | | 1987 | 9.5775805894367 | | 1986 | 9.7745602563702 | | 1985 | 10.048109128555 | | 1984 | 9.2163002838202 | | 1983 | 9.8508381608037 | | 1982 | 9.939695142277 | | 1981 | 10.008973562504 | | 1980 | 10.645878585472 | | 1979 | 12.213802614772 | | 1978 | 11.717492758288 | | 1977 | 12.529295375604 | | 1976 | 17.241014575533 | | 1975 | 15.2466367713 | | 1974 | 17.509986684421 | | 1973 | 18.665158371041 | | 1972 | 18.355184743743 | | 1971 | 17.152961980548 | | 1970 | 18.508026440038 | | 1968 | 24.063818565401 | | 1967 | 23.492462311558 | | 1966 | 19.494261119082 | | 1965 | 13.409904534606 | | 1964 | 14.842175957018 | | 1963 | 17.11851278079 | | 1962 | 18.145500420521 | | 1961 | 16.50707743547 | | 1960 | 20.396744659207 |
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