| Year | Value | | 2020 | 19.526415529672 | | 2019 | 17.325026913276 | | 2018 | 16.169389648734 | | 2017 | 15.618546886849 | | 2016 | 13.79078804777 | | 2015 | 17.42518020905 | | 2014 | 15.471429162813 | | 2013 | 12.966093427556 | | 2012 | 12.545488422232 | | 2011 | 14.468855898354 | | 2010 | 11.501116695257 | | 2009 | 10.736752853074 | | 2008 | 12.202005084762 | | 2007 | 12.878762192928 | | 2006 | 11.929599490249 | | 2005 | 12.626591223088 | | 2004 | 17.384699025871 | | 2003 | 17.548643369504 | | 2002 | 16.146622741677 | | 2001 | 11.991031503102 | | 2000 | 13.541094159317 | | 1999 | 5.2819579817615 | | 1998 | 6.9948322880639 | | 1997 | 5.1575252293085 | | 1996 | 11.899595768877 | | 1995 | 16.74161751645 | | 1994 | 14.25111010402 | | 1993 | 2.7622594077183 | | 1992 | 10.205411257183 | | 1991 | 14.004376713382 | | 1990 | 20.951883031436 | | 1989 | 12.942474871682 | | 1988 | 12.757447039796 | | 1987 | 12.702444977863 | | 1986 | 13.528309106034 | | 1985 | 13.205840699594 | | 1984 | 17.726257745099 | | 1983 | 22.178886274759 | | 1982 | 16.140365118664 | | 1981 | 19.80004703599 | | 1980 | 27.229465033715 | | 1979 | 29.444222960167 | | 1978 | 43.398191463571 | | 1977 | 27.05970246841 | | 1976 | 31.044656493862 | | 1975 | 23.257976294178 | | 1974 | 60.37381860538 |
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