| Year | Value | | 2021 | 23.403272822451 | | 2020 | 21.310824917988 | | 2019 | 21.719529653806 | | 2018 | 19.636605368842 | | 2017 | 17.399154611799 | | 2016 | 17.402289432324 | | 2015 | 13.862030051625 | | 2014 | 18.602178763247 | | 2013 | 16.283155949463 | | 2012 | 13.533167737181 | | 2011 | 12.125649003919 | | 2010 | 11.387262420436 | | 2009 | 11.454386753593 | | 2008 | 12.641740750267 | | 2007 | 14.045190406809 | | 2006 | 12.464865639685 | | 2005 | 12.190929724023 | | 2004 | 13.256865781636 | | 2003 | 11.366391004075 | | 2002 | 13.613288752564 | | 2001 | 13.278921809743 | | 2000 | 12.683879166684 | | 1999 | 12.456530472493 | | 1998 | 15.798575236333 | | 1997 | 16.187851175992 | | 1996 | 15.243891330184 | | 1995 | 15.897111537944 | | 1994 | 14.506101473549 | | 1993 | 9.6853133374998 | | 1992 | 9.5752064164883 | | 1991 | 10.756541390665 | | 1990 | 9.688373737769 | | 1989 | 4.514204625127 | | 1988 | 0.7704325961169 | | 1987 | 4.3578626826789 | | 1986 | 4.1373740514115 | | 1985 | 2.6342234863317 | | 1984 | 7.5166133285106 | | 1983 | 8.19251643322 | | 1982 | 13.221546310706 | | 1981 | -3.2515592353955 | | 1980 | 4.1724305986479 | | 1979 | 12.641387570594 | | 1978 | 10.504054229568 | | 1977 | 13.615100526116 | | 1976 | 11.215814822317 | | 1975 | 10.149334631432 | | 1974 | 11.101890635744 |
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