| Year | Value | | 2021 | 3.2244942028988 | | 2020 | 3.3539787380376 | | 2019 | 3.2456940772769 | | 2018 | 3.6938839465372 | | 2017 | 3.9394877720871 | | 2016 | 4.2840208806601 | | 2015 | 4.448451081503 | | 2014 | 4.7430517161152 | | 2013 | 4.6788942487823 | | 2012 | 5.075956674486 | | 2011 | 5.3364332750779 | | 2010 | 5.1469796832699 | | 2009 | 5.255486146625 | | 2008 | 6.5636089331217 | | 2007 | 6.7024550929835 | | 2006 | 6.5781678732171 | | 2005 | 6.3202551565437 | | 2004 | 6.2602696067401 | | 2003 | 5.8370750481078 | | 2002 | 6.3451458380358 | | 2001 | 6.4957264957265 | | 2000 | 6.9011950850025 | | 1999 | 7.3422454893896 | | 1998 | 10.969800367802 | | 1997 | 12.564511335081 | | 1996 | 11.308203991131 | | 1995 | 19.381184231192 | | 1994 | 20.51376146789 | | 1993 | 21.009940842837 | | 1992 | 20.955349376631 | | 1991 | 27.254208212716 | | 1990 | 27.464479273473 | | 1989 | 38.564330991948 | | 1988 | 39.789964994166 | | 1987 | 23.788653518383 | | 1986 | 21.719180177424 | | 1985 | 20.441568328892 | | 1984 | 17.757398916215 | | 1983 | 8.739837398374 | | 1982 | 8.0226904376013 | | 1981 | 12.729844413013 | | 1980 | 17.047817047817 | | 1979 | 18.491484184915 | | 1978 | 17.277486910995 | | 1977 | 16.358024691358 | | 1976 | 15.079365079365 | | 1975 | 15.311004784689 | | 1974 | 20.555555555556 | | 1973 | 21.259842519685 | | 1972 | 22.340425531915 |
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