| Year | Value | | 2021 | 3.3662224640046 | | 2020 | 3.0326454174822 | | 2019 | 4.2422183850195 | | 2018 | 4.8517945792174 | | 2017 | 4.5051398006514 | | 2016 | 5.0626631130637 | | 2015 | 4.4868843445464 | | 2014 | 4.6781752283676 | | 2013 | 4.1653981390034 | | 2012 | 3.9893763445776 | | 2011 | 4.0586566056442 | | 2010 | 4.1265531237702 | | 2009 | 4.0200420014075 | | 2008 | 4.001314491041 | | 2007 | 4.6323984040218 | | 2006 | 4.5760378491341 | | 2005 | 4.4789317269169 | | 2004 | 4.5142450688615 | | 2003 | 4.6756911087972 | | 2002 | 4.7533009034051 | | 2001 | 5.4478962449103 | | 2000 | 5.3921003038345 | | 1999 | 4.7704392922514 | | 1998 | 4.0232522563867 | | 1997 | 4.3051546391753 | | 1996 | 5.6618524268531 | | 1995 | 6.7119279891114 | | 1994 | 6.9977131656321 | | 1993 | 6.0112787684804 | | 1992 | 7.9839983156122 | | 1991 | 5.1548886737657 | | 1990 | 7.5519513431323 | | 1988 | 1.1865781710915 | | 1987 | 1.1218465783679 | | 1986 | 5.2638395500296 | | 1985 | 6.2869760151022 | | 1984 | 10.968726704767 | | 1983 | 13.455620587703 | | 1982 | 9.2864513056592 | | 1981 | 9.5833333333333 | | 1980 | 12.357142857143 | | 1979 | 10.071748878924 | | 1978 | 8.4896010910331 | | 1977 | 4.7322844249299 | | 1976 | 12.124908514272 | | 1973 | 3.8152893143742 | | 1972 | 5.0903377847604 | | 1971 | 4.0007408779404 | | 1970 | 4.3362104397863 | | 1969 | 4.644030668127 | | 1968 | 4.844371635853 | | 1967 | 4.8429319371728 | | 1966 | 4.163434186708 | | 1965 | 3.8887311950043 | | 1964 | 3.65625 |
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