| Year | Value | | 2022 | 2.6857168430026 | | 2021 | 2.155261673871 | | 2020 | 2.298611835872 | | 2019 | 2.1616100034463 | | 2018 | 2.1620849382097 | | 2017 | 2.2843972908031 | | 2016 | 2.2946202213193 | | 2015 | 2.2322028897808 | | 2014 | 2.2577259110872 | | 2013 | 2.2071459277433 | | 2012 | 2.2785369606388 | | 2011 | 2.2798532862722 | | 2010 | 2.4098317782433 | | 2009 | 2.5569292990818 | | 2008 | 2.278643989835 | | 2007 | 2.2478432812077 | | 2006 | 2.3580172976363 | | 2005 | 2.4701947313411 | | 2004 | 2.4770701914167 | | 2003 | 2.4410901148586 | | 2002 | 2.5453526572815 | | 2001 | 2.6252814578902 | | 2000 | 2.589159217257 | | 1999 | 2.6959404287199 | | 1998 | 2.3832447408034 | | 1997 | 2.3977343462883 | | 1996 | 2.3491291658276 | | 1995 | 2.4853789737043 | | 1994 | 2.6050480829055 | | 1993 | 2.6131524187027 | | 1992 | 2.5428772004473 | | 1991 | 2.8184201971219 | | 1990 | 3.0499071829874 | | 1989 | 3.53113031769 | | 1988 | 3.9959611663827 | | 1987 | 4.1271563466482 | | 1986 | 4.1788717265891 | | 1985 | 4.0349963511145 | | 1984 | 3.9403941089847 | | 1983 | 4.1415418387768 | | 1982 | 4.083863632073 | | 1981 | 3.9137428923994 | | 1980 | 3.6288139028525 | | 1979 | 3.9475606691778 | | 1978 | 5.1178341047966 | | 1977 | 5.4227658994351 | | 1976 | 5.7831756295488 | | 1975 | 5.7926298509852 | | 1974 | 4.5931571651444 | | 1973 | 3.4865689318859 | | 1972 | 4.1344461455305 | | 1971 | 4.3319548248935 | | 1970 | 4.3458724880674 | | 1969 | 4.2710791929051 | | 1968 | 4.0627623329152 | | 1967 | 3.7860462060894 | | 1966 | 3.6639171237873 | | 1965 | 3.7555756856013 | | 1964 | 3.2770056122473 | | 1963 | 3.3218904947263 | | 1962 | 2.5206869650848 | | 1961 | 2.1138498083479 | | 1960 | 2.024516412964 |
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