| Year | Value | | 2020 | 0.72711641266792 | | 2019 | 1.5046235385811 | | 2018 | 0.98919111919048 | | 2017 | 0.29625288927919 | | 2016 | 0.14181622573231 | | 2015 | 0.84382698463833 | | 2014 | 0.79925037268749 | | 2013 | 0.046411646880599 | | 2012 | 1.3567714780886 | | 2011 | 1.1312313652648 | | 2010 | 2.99936630549 | | 2009 | 0.006263571070648 | | 2008 | 6.359921833363 | | 2007 | 3.6322212931781 | | 2006 | 2.2023913263168 | | 2005 | 1.6819007987755 | | 2004 | 2.394109820503 | | 2003 | 1.4533680043725 | | 2002 | 0.17005533019768 | | 2001 | 1.3038940346596 | | 2000 | 0.85694680535849 | | 1999 | 1.1794778739417 | | 1998 | 0.99914745791075 | | 1997 | 2.4362971547967 | | 1996 | 1.6771645048591 | | 1995 | 1.3175374577497 | | 1994 | 0.015136735174436 | | 1993 | 1.569026042245 | | 1992 | 5.4718255934418 | | 1991 | 5.5570615342913 | | 1990 | 3.1909232322682 | | 1989 | 6.2185214977786 | | 1988 | 2.9245916312374 | | 1987 | 4.0245804334164 | | 1986 | 2.7742336767571 | | 1985 | 3.7399799806424 | | 1984 | 2.2179942499574 | | 1983 | 4.1479524438573 | | 1982 | 4.3945940976374 | | 1981 | 13.266687493491 | | 1978 | 7.7125790583273 | | 1977 | 9.50365525202 | | 1976 | 10.902496266269 | | 1975 | 19.90278843694 | | 1974 | 34.37607425232 | | 1973 | 12.099116723085 | | 1972 | 3.6879955745474 | | 1971 | 3.6374060388718 | | 1970 | 12.393498961891 | | 1969 | 4.2157886885082 | | 1968 | 5.3699190185797 | | 1967 | 0.95245654425543 |
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