Year | Value | 2020 | 18.515468083928 | 2019 | 17.915114357118 | 2018 | 18.364472985242 | 2017 | 18.077462198057 | 2016 | 21.732247876125 | 2015 | 17.095615857288 | 2014 | 17.275289380084 | 2013 | 17.859739726467 | 2012 | 17.791874450606 | 2011 | 18.198773485894 | 2010 | 18.136419018586 | 2009 | 18.122699047162 | 2008 | 17.800201322153 | 2007 | 18.110942594677 | 2006 | 18.832306418654 | 2005 | 17.896921966574 | 2004 | 17.846204617424 | 2003 | 17.206005898569 | 2002 | 15.777676218729 | 2001 | 15.305981583975 | 2000 | 14.699137038658 | 1999 | 15.278285168979 | 1998 | 16.982841774564 | 1997 | 16.780792455572 | 1996 | 17.35110223493 | 1995 | 16.878758028337 | 1994 | 19.034409383504 | 1993 | 19.389494107735 | 1992 | 17.240857474406 | 1991 | 16.79324031609 | 1990 | 16.202586399924 | 1989 | 14.532178094581 | 1988 | 15.189103076773 | 1987 | 15.965446347106 | 1986 | 16.763228438687 | 1985 | 15.949752648996 | 1984 | 13.869256647092 | 1983 | 15.121297642464 | 1982 | 14.005553777347 | 1981 | 16.739064806118 | 1980 | 15.82577762353 | 1979 | 14.447000719507 | 1978 | 16.023072683687 | 1977 | 15.847694761892 | 1976 | 15.523334544374 | 1975 | 12.236126911895 | 1974 | 12.710355266673 | 1973 | 10.447321954091 | 1972 | 13.680949922561 |
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