| Year | Value | | 2021 | 9.9494750103916 | | 2020 | 9.9121368915725 | | 2019 | 9.8788585677536 | | 2018 | 9.8477573106439 | | 2017 | 9.7786818067789 | | 2016 | 9.6800545967964 | | 2015 | 9.5725336095643 | | 2014 | 9.4939254373477 | | 2013 | 9.4214961652311 | | 2012 | 9.2701881225864 | | 2011 | 9.0455296503218 | | 2010 | 8.9128606708662 | | 2009 | 8.7592234186512 | | 2008 | 8.577066867711 | | 2007 | 8.4576871969682 | | 2006 | 8.3106952442652 | | 2005 | 8.225106963839 | | 2004 | 8.0567729497396 | | 2003 | 8.0245296841702 | | 2002 | 7.7780332904648 | | 2001 | 7.7142377707326 | | 2000 | 7.6748626949753 | | 1999 | 7.6728202620691 | | 1998 | 7.6293299957901 | | 1997 | 7.6161945312332 | | 1996 | 7.5642166908711 | | 1995 | 7.442407647745 | | 1994 | 7.2932925049979 | | 1993 | 7.2833297312571 | | 1992 | 7.1178282669617 | | 1991 | 7.1177906792757 | | 1990 | 7.067057716753 | | 1989 | 6.978542999613 | | 1988 | 6.9580566011483 | | 1987 | 6.8988915263674 | | 1986 | 6.8825425174474 | | 1985 | 6.8564417939703 | | 1984 | 6.7750133595077 | | 1983 | 6.7448349104995 | | 1982 | 6.7286699012605 | | 1981 | 6.6992572680645 | | 1980 | 6.6996617232427 | | 1979 | 6.6864654934833 | | 1978 | 6.6887844881919 | | 1977 | 6.606888607714 | | 1976 | 6.6018229549212 | | 1975 | 6.5595108008375 | | 1974 | 6.5470438051921 | | 1973 | 6.560638140334 | | 1972 | 6.5787386472775 | | 1971 | 6.5284152598412 | | 1970 | 6.5305783698743 | | 1969 | 6.5108617462395 | | 1968 | 6.4786388657466 | | 1967 | 6.4267242249527 | | 1966 | 6.4018111645717 | | 1965 | 6.402656517918 | | 1964 | 6.3582506039055 | | 1963 | 6.3137949632256 | | 1962 | 6.2115569349954 | | 1961 | 6.1725215010651 |
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