| Year | Value | | 2018 | 12.017427748056 | | 2017 | 11.3874439799 | | 2016 | 11.147738418282 | | 2015 | 10.569709051864 | | 2014 | 9.9846541903527 | | 2013 | 11.001625693939 | | 2012 | 10.836771718735 | | 2011 | 10.177387199726 | | 2010 | 10.388039790504 | | 2009 | 9.8097254064651 | | 2008 | 10.977172152143 | | 2007 | 12.108346990341 | | 2006 | 11.129337053899 | | 2005 | 10.080931526437 | | 2004 | 9.5708170803596 | | 2003 | 9.108084322144 | | 2002 | 8.6762739309022 | | 2001 | 8.0794544445696 | | 2000 | 8.8101434963631 | | 1999 | 8.6383000275708 | | 1998 | 8.1131435109783 | | 1997 | 9.0085777326938 | | 1996 | 9.2314686908927 | | 1995 | 9.226824690906 | | 1994 | 8.9817720164793 | | 1993 | 8.5352336200357 | | 1992 | 9.8052284422211 | | 1991 | 10.170922420125 | | 1990 | 9.9935913961029 | | 1989 | 10.468344514214 | | 1988 | 10.356738010319 | | 1987 | 10.408309965738 | | 1986 | 10.427308859274 | | 1985 | 10.07541854281 | | 1984 | 9.3065398643845 | | 1983 | 9.2684974061858 | | 1982 | 9.2691213516116 | | 1981 | 9.213690314791 | | 1980 | 9.026071738655 | | 1979 | 9.6890489231657 | | 1978 | 9.3022745302713 | | 1977 | 8.5970942063643 | | 1976 | 9.1818379905321 | | 1975 | 9.0245029081278 | | 1974 | 8.181120885498 |
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