| Year | Value | | 2020 | 3.9740159711055 | | 2015 | 4.4343624327463 | | 2014 | 4.3356182017805 | | 2013 | 4.6741180000476 | | 2012 | 5.5029323639415 | | 2011 | 5.3007598926526 | | 2010 | 5.8025632176187 | | 2009 | 5.2903872042874 | | 2008 | 7.0284815210304 | | 2007 | 7.5225058215985 | | 2006 | 7.4849927946327 | | 2005 | 8.1564798035016 | | 2004 | 8.3359317270277 | | 2003 | 9.3345693853018 | | 1981 | 28.636818766986 | | 1980 | 21.401000714796 | | 1979 | 25.364077669903 | | 1978 | 26.297507151614 | | 1977 | 26.349496797804 | | 1976 | 25.508905852417 | | 1975 | 24.75922953451 | | 1974 | 28.205128205128 | | 1973 | 28.441295546559 | | 1972 | 30.163360560093 |
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