| Year | Value | | 2021 | 3.5564943548992 | | 2020 | 3.5525743684576 | | 2019 | 3.8886506263588 | | 2018 | 4.1209371235722 | | 2017 | 3.9076573734379 | | 2016 | 3.7840509755675 | | 2015 | 4.0759382857018 | | 2014 | 3.9156970252456 | | 2013 | 4.6890572558943 | | 2012 | 4.2144058348523 | | 2011 | 4.4310160019632 | | 2010 | 3.8140456320749 | | 2009 | 3.1002266387463 | | 2008 | 3.4820897261115 | | 2007 | 4.4315876448752 | | 2006 | 4.6116243555648 | | 2005 | 4.3199256868516 | | 2004 | 3.5110373582853 | | 2003 | 2.6399366127089 | | 2002 | 2.8906691134952 | | 2001 | 3.0286622189901 | | 2000 | 3.1035698749111 | | 1999 | 2.7851418063681 | | 1998 | 2.8282740190917 | | 1997 | 3.0149153243036 | | 1996 | 4.2099259188842 | | 1995 | 4.473149239446 | | 1994 | 0.76547322715278 | | 1993 | 3.7188180143981 | | 1992 | 2.6096297232402 | | 1991 | 4.0759309579395 | | 1990 | 4.0160351963877 | | 1989 | 9.5380120518595 | | 1988 | 6.2971985383678 | | 1987 | 4.1303514376997 | | 1986 | 4.0628706998814 | | 1985 | 3.0167224080268 | | 1984 | 2.5925151578507 | | 1983 | 3.48250948594 | | 1982 | 5.2398131172748 | | 1981 | 4.9186204918621 | | 1980 | 3.4939078512063 | | 1979 | 5.0811272416738 | | 1978 | 6.3736828578772 | | 1977 | 8.2851637764933 | | 1976 | 4.8784692294432 | | 1975 | 5.7082043343653 | | 1974 | 5.4116028433845 | | 1973 | 4.8382637544927 | | 1972 | 4.8728813559322 |
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