| Year | Value | | 2024 | 2.9900400639546 | | 2023 | 3.0097411356824 | | 2022 | 3.1092164150158 | | 2021 | 2.8366814338847 | | 2020 | 2.9826494148889 | | 2019 | 2.5148341589022 | | 2018 | 2.5511370748365 | | 2017 | 2.4066827050972 | | 2016 | 2.0394816901674 | | 2015 | 2.0401052469457 | | 2014 | 1.8719666565996 | | 2013 | 2.0311189067384 | | 2012 | 1.8646312307317 | | 2011 | 1.8379319358581 | | 2010 | 1.5169945568784 | | 2009 | 1.3968452475291 | | 2008 | 1.632618789985 | | 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 |
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