| Year | Value | | 2015 | 59.838823843735 | | 2014 | 57.721670111155 | | 2013 | 58.248008670468 | | 2012 | 59.479633173387 | | 2011 | 60.056971160503 | | 2010 | 60.470832016727 | | 2009 | 60.949282279369 | | 2008 | 61.763992149386 | | 2007 | 62.333682284144 | | 2006 | 62.610938930877 | | 2005 | 62.698260328046 | | 2004 | 62.025023095246 | | 2003 | 62.366082531711 | | 2002 | 61.932199558869 | | 2001 | 62.125015432732 | | 2000 | 61.658482334351 | | 1999 | 61.181415044422 | | 1998 | 61.035409563474 | | 1997 | 59.101069524066 | | 1996 | 58.297963222094 | | 1995 | 58.138688185111 | | 1994 | 57.200571726841 | | 1993 | 56.303500221203 | | 1992 | 56.16277280758 | | 1991 | 56.676438474767 | | 1990 | 55.378526318654 | | 1989 | 53.848368484157 | | 1988 | 54.039009585517 | | 1987 | 53.671624630111 | | 1986 | 52.868484484123 | | 1985 | 52.251336595809 | | 1984 | 53.595732845683 | | 1983 | 55.050611305935 | | 1982 | 56.202003797848 | | 1981 | 57.472679402238 | | 1980 | 60.698485830284 | | 1979 | 61.993031829824 | | 1978 | 61.944188396005 | | 1977 | 59.918986143696 | | 1976 | 60.840613431623 | | 1975 | 58.390090387945 | | 1974 | 60.778989341023 | | 1973 | 62.488653237862 | | 1972 | 60.611752791208 | | 1971 | 59.364858112308 | | 1970 | 58.995154026942 | | 1969 | 61.943267530809 | | 1968 | 60.140894718301 | | 1967 | 57.510870609259 | | 1966 | 53.291575846325 | | 1965 | 50.42424932725 | | 1964 | 47.402464547611 | | 1963 | 45.88087886372 | | 1962 | 41.867468914775 | | 1961 | 36.834266801572 | | 1960 | 33.98495186599 |
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