| Year | Value | | 2022 | 6.1150555010994 | | 2021 | 2.4107031217436 | | 2020 | 0.079977871310175 | | 2019 | 1.2159494560511 | | 2018 | 1.2450004090527 | | 2017 | 1.8803844018353 | | 2016 | 0.6634271679176 | | 2015 | 0.84382698463833 | | 2014 | 1.2013996448106 | | 2013 | 1.1069855400014 | | 2012 | 2.5984441414709 | | 2011 | 3.4567496723461 | | 2010 | 3.3700254022015 | | 2009 | 0.42153912568727 | | 2008 | 8.0332732765031 | | 2007 | 4.0342298288507 | | 2006 | 4.2519532197796 | | 2005 | 3.7334291646702 | | 2004 | 2.394109820503 | | 2003 | 2.2354366437455 | | 2002 | 2.1735779250809 | | 2001 | 2.6274131420333 | | 2000 | 2.4358130348914 | | 1999 | 2.6213952458275 | | 1998 | 2.6986029585834 | | 1997 | 2.9959840302924 | | 1996 | 2.2363630400572 | | 1995 | 2.9232080473239 | | 1994 | 2.5693430464429 | | 1993 | 2.7228976697059 | | 1992 | 5.4718255934418 | | 1991 | 5.5570615342913 | | 1990 | 4.2675566285597 | | 1989 | 5.3876582278481 | | 1988 | 4.0000000000001 | | 1987 | 4.0245804334164 | | 1986 | 2.1902582543313 | | 1985 | 3.9151052508279 | | 1984 | 3.6887445276091 | | 1983 | 4.9960967998049 | | 1982 | 6.8390325271629 | | 1981 | 12.739718114509 | | 1980 | 17.470304279577 | | 1979 | 14.782239267266 | | 1978 | 9.4823825910176 | | 1977 | 9.7311327685374 | | 1976 | 9.931595652813 | | 1975 | 17.177603627135 | | 1974 | 27.158189455022 | | 1973 | 13.440600697174 | | 1972 | 6.8308113190237 | | 1971 | 4.6149718266825 | | 1970 | 7.2842163736852 | | 1969 | 5.8352318660549 | | 1968 | 5.3699190185797 | | 1967 | 3.2071790871853 |
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