| Year | Value | | 2021 | 0.91591875705814 | | 2020 | 0.97836032087155 | | 2019 | 1.6301514964897 | | 2018 | 1.7789240269324 | | 2017 | 3.2858039010389 | | 2016 | 2.2785072155193 | | 2015 | 2.7129402331584 | | 2009 | 5.4539187582501 | | 2008 | 4.9789217781451 | | 2007 | 4.0832306034626 | | 2006 | 3.6405121940266 | | 2005 | 3.311281458924 | | 2004 | 5.7826201606638 | | 2003 | 2.2561843943982 | | 2002 | 3.2738003511599 | | 2001 | 2.9440449110298 | | 2000 | 4.7914208815769 | | 1999 | 4.0097860954662 | | 1998 | 2.3796607387889 | | 1997 | 0.95430490661445 | | 1996 | 1.2693140932039 | | 1995 | 1.667984222442 | | 1994 | 3.8978350650433 | | 1993 | 4.9211184229432 | | 1992 | 4.3893635177683 | | 1991 | 4.9923013458193 | | 1990 | 4.8133247359483 | | 1988 | 3.5519651214977 | | 1987 | 2.5527131145293 | | 1986 | 3.2272479022889 | | 1985 | 3.3996981240566 | | 1984 | 4.0845194942976 | | 1983 | 3.2686859882327 | | 1982 | 2.4404761904762 | | 1981 | 1.7818068146296 | | 1980 | 2.3027046680267 | | 1979 | 2.5931520644512 | | 1978 | 2.0406908845043 | | 1977 | 2.6156034273424 | | 1976 | 2.6666666666667 | | 1975 | 2.2510822510822 | | 1974 | 2.5678358702846 | | 1973 | 3.1621187800963 | | 1972 | 4.1917502787068 | | 1971 | 4.6033653846154 | | 1970 | 4.9540078843627 | | 1969 | 5.1972555746141 | | 1968 | 3.75 | | 1967 | 3.6546184738956 | | 1966 | 3.5251509054326 | | 1965 | 4.1153039832285 | | 1964 | 3.1422413793103 | | 1963 | 2.1491228070175 | | 1962 | 2.0595238095238 | | 1961 | 1.8449612403101 | | 1960 | 1.9174041297935 |
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