| Year | Value | | 2015 | 54.703504516089 | | 2014 | 51.875363516551 | | 2013 | 52.355991257053 | | 2012 | 53.008011734095 | | 2011 | 53.972381190388 | | 2010 | 54.586503403361 | | 2009 | 54.466205510635 | | 2008 | 55.277124237978 | | 2007 | 55.714053389575 | | 2006 | 55.561292513147 | | 2005 | 55.198049354579 | | 2004 | 54.519321883261 | | 2003 | 55.001053880712 | | 2002 | 54.302587756257 | | 2001 | 54.461077753416 | | 2000 | 53.965649820653 | | 1999 | 53.638768929668 | | 1998 | 53.490035980478 | | 1997 | 51.137185107836 | | 1996 | 50.958574629188 | | 1995 | 50.495838706009 | | 1994 | 49.488633181053 | | 1993 | 48.909033783704 | | 1992 | 48.704790645196 | | 1991 | 49.241473459008 | | 1990 | 48.356306808062 | | 1989 | 46.110201943886 | | 1988 | 45.565994650088 | | 1987 | 45.335119531958 | | 1986 | 44.569942134043 | | 1985 | 44.138935175859 | | 1984 | 44.615657963855 | | 1983 | 45.890481103352 | | 1982 | 47.192590185989 | | 1981 | 49.544038441147 | | 1980 | 51.590754417798 | | 1979 | 52.124732008167 | | 1978 | 51.600104800296 | | 1977 | 50.080820595267 | | 1976 | 50.965181702965 | | 1975 | 48.610520230978 | | 1974 | 51.034641904786 | | 1973 | 52.821983137129 | | 1972 | 50.983986533235 | | 1971 | 49.893050981142 | | 1970 | 53.555279031993 | | 1969 | 55.023471094476 | | 1968 | 53.120273966303 | | 1967 | 50.678293167296 | | 1966 | 47.050490857674 | | 1965 | 44.480228912214 | | 1964 | 42.047720788168 | | 1963 | 40.963000537116 | | 1962 | 37.421739003954 | | 1961 | 32.703229537125 | | 1960 | 30.244338738166 |
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