| Year | Value | | 2021 | 1.70195422988 | | 2020 | 1.8231021385052 | | 2019 | 1.6090307501103 | | 2018 | 1.6047574776129 | | 2017 | 1.6009252103823 | | 2016 | 1.7793854903286 | | 2015 | 1.6537886022339 | | 2014 | 1.7663478903116 | | 2013 | 1.7483078546524 | | 2012 | 1.7687389215328 | | 2011 | 1.7655559440036 | | 2010 | 1.8457095091998 | | 2009 | 1.8330766647452 | | 2008 | 1.7267173578156 | | 2007 | 1.9480300968844 | | 2006 | 2.0181934850185 | | 2005 | 2.049205557953 | | 2004 | 2.3268529662115 | | 2003 | 2.4895443108896 | | 2002 | 2.5280636401296 | | 2001 | 2.458600239978 | | 2000 | 2.3169697537384 | | 1999 | 2.7534948991861 | | 1998 | 2.5378037807483 | | 1997 | 3.0704745264835 | | 1996 | 3.875618161574 | | 1995 | 3.6249383364346 | | 1994 | 3.7779147902492 | | 1993 | 3.9762541481976 | | 1992 | 3.9404453292533 | | 1991 | 4.0807632748885 | | 1990 | 4.0887996734397 | | 1989 | 15.375952550771 | | 1988 | 15.057034220532 | | 1987 | 15.940681904601 | | 1986 | 15.803195962994 | | 1985 | 16.485827637302 | | 1984 | 16.929448158866 | | 1983 | 18.407445708376 | | 1982 | 18.396890666366 | | 1981 | 20.135311278009 | | 1980 | 21.173878205128 | | 1979 | 22.036262203626 | | 1978 | 21.636876763876 | | 1977 | 22.50828644107 | | 1976 | 21.543139490688 | | 1975 | 25.972074938141 | | 1974 | 24.864672996717 | | 1973 | 29.445977917981 | | 1972 | 30.551197573476 |
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