| Year | Value | | 2021 | 20.649554782498 | | 2020 | 19.866996025845 | | 2019 | 20.668478849986 | | 2018 | 20.824373988058 | | 2017 | 20.733889819311 | | 2016 | 20.813449921386 | | 2015 | 20.470705111526 | | 2014 | 20.631521965036 | | 2013 | 20.643497242085 | | 2012 | 20.135596929918 | | 2011 | 20.017878968474 | | 2010 | 18.601276688476 | | 2009 | 18.882842632891 | | 2008 | 20.491081769747 | | 2007 | 21.065151385563 | | 2006 | 21.420267558142 | | 2005 | 21.792855538081 | | 2004 | 21.812444097305 | | 2003 | 22.05813547371 | | 2002 | 22.467438007624 | | 2001 | 21.88995215311 | | 2000 | 24.240336553261 | | 1999 | 22.387248258691 | | 1998 | 22.70946428275 | | 1997 | 22.615899717527 | | 1996 | 21.910602157068 | | 1995 | 20.70239170362 | | 1994 | 23.108627077661 | | 1993 | 22.41930067482 | | 1992 | 24.884005000447 | | 1991 | 24.439399806758 | | 1990 | 24.418581524101 | | 1989 | 24.648281244068 | | 1988 | 24.189221935268 | | 1987 | 22.665978192783 | | 1986 | 23.974418573301 | | 1985 | 23.477873721143 | | 1984 | 22.738403615661 | | 1983 | 22.337591062396 | | 1982 | 22.162314389838 | | 1981 | 22.527559288674 | | 1980 | 21.458114954345 | | 1979 | 21.187765129443 | | 1978 | 22.418678766294 | | 1977 | 24.220504417349 | | 1976 | 24.702069662192 | | 1975 | 22.159466698667 | | 1974 | 21.348280544914 | | 1973 | 22.003720048907 | | 1972 | 21.765519843679 |
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