| Year | Value | | 2021 | 1.0328881539891 | | 2020 | 0.82272267956351 | | 2019 | 0.8753270126682 | | 2018 | 0.99293905379669 | | 2017 | 0.98120769022475 | | 2016 | 1.0449030263359 | | 2015 | 1.1622733404449 | | 2014 | 1.336918828199 | | 2013 | 1.2630305558452 | | 2012 | 1.2681922076321 | | 2011 | 1.2731292719093 | | 2010 | 1.3903417529449 | | 2009 | 1.1607229636668 | | 2008 | 1.7738280291557 | | 2007 | 1.6350610380335 | | 2006 | 1.9626226639165 | | 2005 | 2.3051503183687 | | 2004 | 2.6208045199426 | | 2002 | 5.6089270068679 | | 2001 | 7.0611680044284 | | 2000 | 8.0850762788929 | | 1988 | 14.08492170803 | | 1987 | 13.647107558848 | | 1986 | 11.459048665476 | | 1985 | 14.775720483421 | | 1984 | 12.696690179056 | | 1983 | 8.4673416716299 | | 1982 | 5.1513604774432 | | 1981 | 7.8914327917282 | | 1980 | 6.9173350642125 | | 1979 | 7.7186963979417 | | 1978 | 10.193507454795 | | 1977 | 12.036872894877 | | 1976 | 11.75996778091 | | 1975 | 13.48897535668 | | 1974 | 11.464968152866 | | 1973 | 19.047619047619 |
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