| Year | Value | | 2021 | 2.6290696376295 | | 2020 | 2.6605165134839 | | 2019 | 2.7003524466009 | | 2018 | 2.7830485264552 | | 2017 | 2.8380068386617 | | 2016 | 2.8711594084609 | | 2015 | 2.9517489321903 | | 2014 | 2.9745705168866 | | 2013 | 2.9707275231148 | | 2012 | 2.9530281308481 | | 2011 | 2.9201682970947 | | 2010 | 2.9013920788684 | | 2009 | 2.8823700812433 | | 2008 | 2.8896199590847 | | 2007 | 2.9061337155181 | | 2006 | 2.9171126468327 | | 2005 | 2.940256016939 | | 2004 | 2.9757178554421 | | 2003 | 3.0242099930027 | | 2002 | 3.0732973824874 | | 2001 | 3.1531546706119 | | 2000 | 3.2304517968239 | | 1999 | 3.3198275813287 | | 1998 | 3.4270396508624 | | 1997 | 3.5422209455161 | | 1996 | 3.6894692805813 | | 1995 | 3.8893511158444 | | 1994 | 4.108297354744 | | 1993 | 4.3041811235336 | | 1992 | 4.5044067751447 | | 1991 | 4.6857558078702 | | 1990 | 4.8816134636717 | | 1989 | 5.0500065428243 | | 1988 | 5.2447891123281 | | 1987 | 5.4337659285973 | | 1986 | 5.6170456399485 | | 1985 | 5.7846052767782 | | 1984 | 5.9176024823236 | | 1983 | 6.0258578173014 | | 1982 | 6.1102664773588 | | 1981 | 6.1794490960072 | | 1980 | 6.2588817667718 | | 1979 | 6.3210344791304 | | 1978 | 6.3112305700231 | | 1977 | 6.3308831195631 | | 1976 | 6.3655297616755 | | 1975 | 6.3856460382484 | | 1974 | 6.4294223586251 | | 1973 | 6.4840108451183 | | 1972 | 6.5645964466598 | | 1971 | 6.6579594761366 | | 1970 | 6.7436329507071 | | 1969 | 6.818318037738 | | 1968 | 6.8659283108899 | | 1967 | 6.9102029928981 | | 1966 | 6.942769238803 | | 1965 | 6.9729250457426 | | 1964 | 7.0093729780765 | | 1963 | 7.0343162163815 | | 1962 | 7.0542560470731 | | 1961 | 6.9778551611234 | | 1960 | 6.9502467084801 |
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