| Year | Value | | 2022 | 1.818895384153 | | 2021 | 1.7274253494747 | | 2020 | 1.8901717588298 | | 2019 | 1.8000923561072 | | 2018 | 1.7842568951694 | | 2017 | 1.8608876679056 | | 2016 | 1.9266449883896 | | 2015 | 1.9201192679796 | | 2014 | 1.9228566339345 | | 2013 | 1.905458383676 | | 2012 | 1.8759734944074 | | 2011 | 1.8264237497694 | | 2010 | 1.9334748141338 | | 2009 | 2.0935375576721 | | 2008 | 1.8868116540002 | | 2007 | 1.8700319568501 | | 2006 | 1.9006417184115 | | 2005 | 1.9526286356387 | | 2004 | 1.9909659169528 | | 2003 | 2.0485461045839 | | 2002 | 2.0895578268494 | | 2001 | 2.0371900983329 | | 2000 | 1.9639844683954 | | 1999 | 2.0456666187964 | | 1998 | 1.9191491792554 | | 1997 | 2.0380558969866 | | 1996 | 2.0746374853456 | | 1995 | 2.1723789436836 | | 1994 | 2.2983186722799 | | 1993 | 2.3126080190409 | | 1992 | 2.4832828495415 | | 1991 | 2.3798251778848 | | 1990 | 2.5854767336744 | | 1989 | 2.802278358419 | | 1987 | 3.5182105932698 | | 1986 | 3.5995637997509 | | 1985 | 3.4063591036225 | | 1984 | 3.3335106879443 | | 1983 | 3.5061703084531 | | 1982 | 3.5197731386671 | | 1981 | 3.4021826127137 | | 1980 | 3.1476719436585 | | 1979 | 3.4068173384032 | | 1978 | 4.140533605954 | | 1977 | 4.460974689298 | | 1976 | 4.5733684221274 | | 1975 | 4.4779254299327 | | 1974 | 3.6790261236621 | | 1973 | 3.0154549701445 | | 1969 | 3.4140314149018 | | 1968 | 3.3073947978118 | | 1965 | 3.1594639119647 | | 1964 | 2.9298832956233 | | 1963 | 2.8644160937035 |
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