| Year | Value | | 2021 | 5.6595264937993 | | 2020 | 5.6595264937993 | | 2019 | 5.6595264937993 | | 2018 | 5.6595264937993 | | 2017 | 5.7046223224352 | | 2016 | 5.8060879368658 | | 2015 | 5.9075535512965 | | 2014 | 6.0090191657272 | | 2013 | 6.1104847801578 | | 2012 | 6.2119503945885 | | 2011 | 5.8962795941375 | | 2010 | 5.5467869222097 | | 2009 | 5.0845546786922 | | 2008 | 4.6223224351748 | | 2007 | 4.1600901916573 | | 2006 | 4.9379932356257 | | 2005 | 5.7046223224352 | | 2004 | 6.2457722660654 | | 2003 | 6.8996617812852 | | 2002 | 7.6662908680947 | | 2001 | 7.3167981961669 | | 2000 | 6.7305524239008 | | 1999 | 6.1555806087937 | | 1998 | 5.8060879368658 | | 1997 | 4.1713641488162 | | 1996 | 3.7204058624577 | | 1995 | 3.7204058624577 | | 1994 | 3.7204058624577 | | 1993 | 3.6076662908681 | | 1992 | 5.8624577226606 | | 1991 | 6.2006764374295 | | 1990 | 7.3280721533258 | | 1989 | 7.891770011274 | | 1988 | 8.6809470124014 | | 1987 | 7.1025930101466 | | 1986 | 7.2153325817362 | | 1985 | 7.1025930101466 | | 1984 | 7.4408117249155 | | 1983 | 7.2153325817362 | | 1982 | 7.5535512965051 | | 1981 | 7.6662908680947 | | 1980 | 8.3427282976325 | | 1979 | 10.935738444194 | | 1978 | 11.048478015784 | | 1977 | 10.822998872604 | | 1976 | 10.597519729425 | | 1975 | 10.597519729425 | | 1974 | 10.710259301015 | | 1973 | 10.710259301015 | | 1972 | 12.965050732807 | | 1971 | 14.656144306652 | | 1970 | 17.136414881623 | | 1969 | 18.714768883878 | | 1968 | 18.489289740699 | | 1967 | 19.165727170237 | | 1966 | 21.08229988726 | | 1965 | 24.23900789177 | | 1964 | 21.533258173619 | | 1963 | 23.900789177001 | | 1962 | 24.013528748591 | | 1961 | 25.253664036077 |
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