Year | Value | 2021 | 19.243789581915 | 2020 | 18.884351336537 | 2019 | 19.219240822528 | 2018 | 19.164091542219 | 2017 | 19.017342511136 | 2016 | 18.844839962035 | 2015 | 18.929891615371 | 2014 | 19.028614301025 | 2013 | 18.983990279984 | 2012 | 18.513512548928 | 2011 | 18.800792566943 | 2010 | 18.67014996865 | 2009 | 18.484821182803 | 2008 | 19.004397833138 | 2007 | 19.138881826472 | 2006 | 19.030718403963 | 2005 | 18.641350882863 | 2004 | 18.469213242747 | 2003 | 18.746021512373 | 2002 | 18.958694270184 | 2001 | 19.353687425644 | 2000 | 19.757674062236 | 1999 | 19.947398299919 | 1998 | 19.488498347036 | 1997 | 19.148303607975 | 1996 | 18.731963225485 | 1995 | 18.572143192039 | 1994 | 17.815146811338 | 1993 | 17.896935238713 | 1992 | 18.025851979531 | 1991 | 17.928539506056 | 1990 | 17.066030292617 | 1989 | 17.134983694981 | 1988 | 16.955919202006 | 1987 | 16.822079936159 | 1986 | 16.684847423026 | 1985 | 16.16356685236 | 1984 | 16.043754183272 | 1983 | 15.942133706437 | 1982 | 15.536623025878 | 1981 | 15.350934453022 | 1980 | 15.337891725458 | 1979 | 14.921865926906 | 1978 | 14.834090155471 | 1977 | 14.681787029691 | 1976 | 14.530878778782 | 1975 | 13.842294931295 | 1974 | 14.261151645058 | 1973 | 14.178261379951 | 1972 | 13.947373582715 |
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