| Year | Value | | 2021 | 14.377413209855 | | 2020 | 12.389879902419 | | 2019 | 13.302778595113 | | 2018 | 13.024934988758 | | 2017 | 13.030751059434 | | 2016 | 12.910868547509 | | 2015 | 12.811847960954 | | 2014 | 13.386447035733 | | 2013 | 13.503535632633 | | 2012 | 13.026482966778 | | 2011 | 12.29762551272 | | 2010 | 12.207401125726 | | 2009 | 12.635909352309 | | 2008 | 14.16105351716 | | 2007 | 14.88626836531 | | 2006 | 13.994759893545 | | 2005 | 13.698161122021 | | 2004 | 12.52188431883 | | 2003 | 11.745142976413 | | 2002 | 13.216727612614 | | 2001 | 13.380506985606 | | 2000 | 12.164413656635 | | 1999 | 11.529295481643 | | 1998 | 11.431669659002 | | 1997 | 11.408787925253 | | 1996 | 10.344342525509 | | 1995 | 10.574624839151 | | 1994 | 10.534158020129 | | 1993 | 11.154462736089 | | 1992 | 10.870335377272 | | 1991 | 7.5908176979999 | | 1990 | 10.29678928124 | | 1989 | 12.384591795815 | | 1988 | 11.700887353313 | | 1987 | 11.244621400132 | | 1986 | 11.281895538421 | | 1985 | 9.6912455896776 | | 1984 | 9.022770398482 | | 1983 | 8.4571502939071 | | 1982 | 7.9783245034715 | | 1981 | 9.7126155555262 | | 1980 | 10.307189445817 | | 1979 | 11.031497781334 | | 1978 | 11.350963163231 | | 1977 | 12.537332955462 | | 1976 | 13.263317727445 | | 1975 | 16.081351411425 | | 1974 | 14.420480568753 | | 1973 | 13.43824633231 | | 1972 | 13.852269296568 |
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