| Year | Value | | 2021 | 4.583450751987 | | 2020 | 4.6139730937994 | | 2019 | 4.5283863250259 | | 2018 | 4.5856295792047 | | 2017 | 4.5952201628484 | | 2016 | 4.5391788072069 | | 2015 | 4.4743768266439 | | 2014 | 4.4435629654395 | | 2013 | 4.4568405480069 | | 2012 | 4.4367347918617 | | 2011 | 4.2473147871367 | | 2010 | 4.2430701023887 | | 2009 | 4.2311238930074 | | 2008 | 4.2203561280259 | | 2007 | 4.2824003692218 | | 2006 | 4.2439982673234 | | 2005 | 4.2374610460126 | | 2004 | 4.2238931630114 | | 2003 | 4.2762723940658 | | 2002 | 4.1295807252007 | | 2001 | 4.1121480100072 | | 2000 | 4.12787039805 | | 1999 | 4.170068510798 | | 1998 | 4.1945418890931 | | 1997 | 4.2171505112137 | | 1996 | 4.2460958366538 | | 1995 | 4.2001412415785 | | 1994 | 4.1234819175148 | | 1993 | 4.1403567339834 | | 1992 | 3.9651951363527 | | 1991 | 3.9042535707724 | | 1990 | 3.851856357875 | | 1989 | 3.8098034068621 | | 1988 | 3.8002672904374 | | 1987 | 3.7916715614568 | | 1986 | 3.7550699189085 | | 1985 | 3.7314867650385 | | 1984 | 3.7092671729624 | | 1983 | 3.6741774971015 | | 1982 | 3.6861233563515 | | 1981 | 3.6717427752347 | | 1980 | 3.6476747340889 | | 1979 | 3.6544337688429 | | 1978 | 3.6625886912092 | | 1977 | 3.6451033621716 | | 1976 | 3.653405220076 | | 1975 | 3.6136591570117 | | 1974 | 3.6088837520225 | | 1973 | 3.5794231766273 | | 1972 | 3.571037356708 | | 1971 | 3.5303363322544 | | 1970 | 3.5085899365174 | | 1969 | 3.4880924891843 | | 1968 | 3.4706806790841 | | 1967 | 3.4785417072728 | | 1966 | 3.4573830519612 | | 1965 | 3.4656114179157 | | 1964 | 3.431522473247 | | 1963 | 3.4036782705973 | | 1962 | 3.3869276684756 | | 1961 | 3.385311382306 |
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