| Year | Value | | 2017 | 2.8356441761773 | | 2016 | 2.7529094282435 | | 2015 | 2.7819703480316 | | 2014 | 2.813975102362 | | 2013 | 2.873015170852 | | 2012 | 2.9160582207932 | | 2011 | 2.95312431095 | | 2010 | 3.0232581898155 | | 2009 | 3.0523381936798 | | 2008 | 3.1024042119933 | | 2007 | 3.1222817538927 | | 2006 | 3.1642532988697 | | 2005 | 3.1901623998134 | | 2004 | 3.2777040478777 | | 2003 | 3.3467193534525 | | 2002 | 3.4145779991686 | | 2001 | 3.4915975237458 | | 2000 | 3.5174638179806 | | 1999 | 3.6294736432806 | | 1998 | 3.7492717056807 | | 1997 | 3.8593052082745 | | 1996 | 3.969789278783 | | 1995 | 4.1892212827297 | | 1994 | 4.3000001907 | | 1993 | 4.5894523688074 | | 1992 | 4.6896294626477 | | 1991 | 4.8997709459409 | | 1990 | 5.0098403270519 | | 1989 | 5.1293701799176 | | 1988 | 5.2382591293567 | | 1987 | 5.3377755404462 | | 1986 | 5.5274460487341 | | 1985 | 5.6273656989702 | | 1984 | 5.8077439353013 | | 1983 | 5.8978802749155 | | 1982 | 5.9880333038263 | | 1981 | 5.9976064256276 | | 1980 | 6.0779077076013 | | 1979 | 6.1679663758523 | | 1978 | 6.2680361962751 | | 1977 | 6.4486256323674 | | 1976 | 6.6291319520312 | | 1975 | 6.8000001907349 | | 1974 | 7.0999999046326 | | 1973 | 7.1999998092651 | | 1972 | 7.4000000953674 | | 1971 | 7.5 | | 1970 | 7.8152225481291 | | 1965 | 8.8000001907349 | | 1960 | 8.9294389688968 |
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