| Year | Value | | 2021 | 5.7503674468402 | | 2020 | 5.6161084001507 | | 2019 | 5.1988664211611 | | 2018 | 5.2269918139928 | | 2017 | 5.2533517011403 | | 2016 | 5.4086497398537 | | 2015 | 5.451670484907 | | 2014 | 5.5194145407258 | | 2013 | 5.6026295857631 | | 2012 | 5.5592977916797 | | 2011 | 5.5016367595618 | | 2010 | 5.5499768093701 | | 2009 | 5.5847541537782 | | 2008 | 5.7629799024259 | | 2007 | 5.8813489301434 | | 2006 | 5.9943683415911 | | 2005 | 6.1408586795044 | | 2004 | 6.2144099559407 | | 2003 | 6.1881049972139 | | 2002 | 6.1173112814687 | | 2001 | 6.2124745406002 | | 2000 | 6.3250637236494 | | 1999 | 6.48496558227 | | 1998 | 7.0027937946408 | | 1997 | 6.8475300278656 | | 1996 | 6.9540523634927 | | 1995 | 7.1264456040839 | | 1994 | 7.0497961711187 | | 1993 | 7.6434979956604 | | 1992 | 8.4612054283654 | | 1991 | 8.5981281802604 | | 1990 | 8.3719472259631 | | 1989 | 8.6307803233132 | | 1988 | 9.9887695692119 | | 1987 | 9.4439098222433 | | 1986 | 9.3311118902373 | | 1985 | 9.9950926009925 | | 1984 | 10.431391099481 | | 1983 | 10.834896875962 | | 1982 | 11.190913433834 | | 1981 | 11.455167728964 | | 1980 | 11.797138189254 | | 1979 | 12.18858581438 | | 1978 | 12.6574189607 | | 1977 | 13.158211861778 | | 1976 | 14.124212706607 | | 1975 | 14.225889051845 | | 1974 | 14.643443631832 | | 1973 | 15.114142675299 | | 1972 | 15.646935860784 | | 1971 | 16.099282891884 | | 1970 | 16.776527269167 | | 1969 | 17.221344095718 | | 1968 | 17.592262388462 | | 1967 | 18.04857212976 | | 1966 | 18.469846895607 | | 1965 | 18.711312867327 | | 1964 | 19.027810073644 | | 1963 | 19.700622114187 | | 1962 | 20.332851312335 | | 1961 | 20.595563602891 | | 1960 | 21.183258230561 |
|