| Year | Value | | 2022 | 3.3888912464173 | | 2021 | 2.842952948435 | | 2020 | 3.0070491441888 | | 2019 | 2.5647662645245 | | 2018 | 2.562919582232 | | 2017 | 2.4182380739402 | | 2016 | 2.0491224850787 | | 2015 | 2.0384448760627 | | 2014 | 1.8790853121937 | | 2013 | 2.0442341087999 | | 2012 | 1.8745243828414 | | 2011 | 1.8477777695829 | | 2010 | 1.5243183303208 | | 2009 | 1.4048472236893 | | 2008 | 1.6374367302099 | | 1998 | 2.0003559136339 | | 1997 | 2.6744136727205 | | 1996 | 2.8186114043175 | | 1995 | 4.7728862675482 | | 1994 | 4.0514711133936 | | 1993 | 5.7586712853016 | | 1992 | 5.509989633067 | | 1991 | 5.8087844926347 | | 1990 | 7.3425311239853 | | 1989 | 7.6696868853164 | | 1988 | 8.1060786818061 | | 1987 | 8.5288833533568 | | 1986 | 7.765395581136 | | 1985 | 9.10141142197 | | 1984 | 10.067763794772 | | 1983 | 7.7185017026107 | | 1981 | 5.8438328912467 | | 1980 | 6.8 | | 1979 | 12.70350271337 | | 1978 | 18.147013782542 | | 1977 | 16.875 | | 1976 | 18.14787154593 | | 1975 | 15.173867228662 | | 1974 | 15.467075038285 | | 1973 | 15.547024952015 | | 1972 | 15.979381443299 |
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