| Year | Value | | 2022 | 4.6313320713891 | | 2021 | 4.7831193161279 | | 2020 | 5.3672234351833 | | 2019 | 5.0357001696531 | | 2018 | 5.4088302642326 | | 2017 | 5.6663643422877 | | 2016 | 5.5253292984481 | | 2015 | 6.5144917136843 | | 2014 | 5.7089486955207 | | 2013 | 5.1770790437151 | | 2012 | 4.5832743075755 | | 2011 | 4.4403297063127 | | 2010 | 4.6195787836316 | | 2009 | 4.8457981669366 | | 2008 | 4.0361671325127 | | 2007 | 4.3046272031637 | | 2006 | 4.3281152600309 | | 2005 | 4.5483654580272 | | 2004 | 4.8636475639851 | | 2003 | 5.2159697729766 | | 2002 | 5.4556806304387 | | 2001 | 5.8414281079967 | | 2000 | 5.8417339546876 | | 1999 | 5.9182260672912 | | 1998 | 6.701876093759 | | 1997 | 6.1686831871153 | | 1996 | 5.699225343139 | | 1995 | 6.2487005882595 | | 1994 | 6.6817100492516 | | 1993 | 6.989810694731 | | 1992 | 8.6673704362992 | | 1991 | 11.441605843789 | | 1989 | 6.7847344882288 | | 1988 | 7.6343242516026 | | 1987 | 8.008987282097 | | 1986 | 6.1687587681549 | | 1985 | 9.1062693343561 | | 1984 | 9.9237874656463 | | 1983 | 9.8573707015264 | | 1982 | 11.112628646191 | | 1981 | 9.7557452432499 | | 1980 | 8.0451887964841 | | 1979 | 8.8683678750229 | | 1978 | 11.005715969934 | | 1977 | 10.914001755612 | | 1974 | 8.120129432884 | | 1973 | 6.2673157382167 | | 1972 | 6.2753422995232 | | 1971 | 6.9296052607266 | | 1970 | 7.8773917285018 | | 1969 | 9.2456754786017 | | 1968 | 8.3310526545272 | | 1967 | 7.1813738859582 | | 1966 | 6.0961984013291 | | 1965 | 5.9042515600813 | | 1964 | 5.1949063858832 | | 1963 | 4.8202960939928 | | 1962 | 5.4075331962274 | | 1961 | 5.0224992758026 | | 1960 | 4.8023398707266 |
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