| Year | Value | | 2021 | 4.2872454448017 | | 2020 | 4.2872454448017 | | 2019 | 4.2872454448017 | | 2018 | 4.2872454448017 | | 2017 | 4.2872454448017 | | 2016 | 4.1800643086817 | | 2015 | 4.1443372633083 | | 2014 | 4.0907466952483 | | 2013 | 4.0371561271883 | | 2012 | 4.0371561271883 | | 2011 | 3.9657020364416 | | 2010 | 3.8585209003215 | | 2009 | 3.6441586280815 | | 2008 | 3.5369774919614 | | 2007 | 3.4297963558414 | | 2006 | 3.394069310468 | | 2005 | 3.2511611289746 | | 2004 | 3.0010718113612 | | 2003 | 2.9653447659879 | | 2002 | 2.8224365844945 | | 2001 | 2.7509824937478 | | 2000 | 2.7152554483744 | | 1999 | 2.7152554483744 | | 1998 | 2.6438013576277 | | 1997 | 2.6080743122544 | | 1996 | 2.6080743122544 | | 1995 | 2.6080743122544 | | 1994 | 2.572347266881 | | 1993 | 2.5366202215077 | | 1992 | 2.5366202215077 | | 1991 | 2.465166130761 | | 1990 | 2.4294390853876 | | 1989 | 2.4294390853876 | | 1988 | 2.3937120400143 | | 1987 | 2.3222579492676 | | 1986 | 2.3579849946409 | | 1985 | 2.2865309038943 | | 1984 | 2.2865309038943 | | 1983 | 2.2150768131476 | | 1982 | 2.2150768131476 | | 1981 | 2.1078956770275 | | 1980 | 2.1078956770275 | | 1979 | 2.1078956770275 | | 1978 | 2.0364415862808 | | 1977 | 2.0007145409075 | | 1976 | 1.9649874955341 | | 1975 | 1.9649874955341 | | 1974 | 1.9649874955341 | | 1973 | 1.9649874955341 | | 1972 | 1.9649874955341 | | 1971 | 1.9649874955341 | | 1970 | 1.9649874955341 | | 1969 | 1.9649874955341 | | 1968 | 1.9649874955341 | | 1967 | 1.9649874955341 | | 1966 | 1.9649874955341 | | 1965 | 1.9649874955341 | | 1964 | 1.9649874955341 | | 1963 | 1.9649874955341 | | 1962 | 1.9649874955341 | | 1961 | 1.9649874955341 |
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