| Year | Value | | 2021 | 2.1577454033998 | | 2020 | 1.7699096377684 | | 2019 | 2.063639033928 | | 2018 | 2.1875709959015 | | 2017 | 1.8813604971358 | | 2016 | 1.9046495600255 | | 2015 | 1.9109952382208 | | 2014 | 1.9828477011239 | | 2013 | 1.9290291452089 | | 2012 | 2.0157822192308 | | 2011 | 2.039394782115 | | 2010 | 1.9683784027244 | | 2009 | 2.6952227579958 | | 2008 | 3.0622055334067 | | 2000 | 5.1283051670082 | | 1999 | 5.5272027936637 | | 1998 | 5.4775105469099 | | 1997 | 5.0830991096223 | | 1996 | 5.4402361403072 | | 1995 | 5.6861374570797 | | 1994 | 8.4004838805635 | | 1993 | 8.9633241467034 | | 1992 | 10.764240360725 | | 1991 | 9.8284515010494 | | 1990 | 8.0379499666904 | | 1989 | 6.6645504564968 | | 1988 | 4.1497739381461 | | 1987 | 6.38092666178 | | 1986 | 6.7164943176001 | | 1985 | 4.9554655870445 | | 1984 | 4.0737893927748 | | 1983 | 3.8556193601313 | | 1982 | 7.1917808219178 | | 1981 | 9.8240469208211 | | 1980 | 8.3487940630798 | | 1979 | 8.3333333333333 | | 1978 | 5.9760956175299 | | 1977 | 5.9782608695652 | | 1976 | 9.6774193548387 | | 1975 | 10.891089108911 | | 1974 | 12.5 | | 1973 | 12.244897959184 | | 1972 | 16.279069767442 |
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