| Year | Value | | 2022 | 27.512879401642 | | 2021 | 27.856504441578 | | 2020 | 22.849418663137 | | 2019 | 24.312877190841 | | 2018 | 24.046573332848 | | 2017 | 22.007148090258 | | 2016 | 21.588847638144 | | 2015 | 21.346897569884 | | 2014 | 19.586610355893 | | 2013 | 18.251243015048 | | 2012 | 17.561333101824 | | 2011 | 17.13138480075 | | 2010 | 18.52283537939 | | 2009 | 18.16965353311 | | 2008 | 20.112198282987 | | 2007 | 22.699073113659 | | 2006 | 23.020350797776 | | 2005 | 21.998910074698 | | 2004 | 24.765897788528 | | 2003 | 23.632482937445 | | 2002 | 23.798405144173 | | 2001 | 23.697074420711 | | 2000 | 23.129881989825 | | 1999 | 24.713396343729 | | 1998 | 25.831077071624 | | 1997 | 21.093575018774 | | 1996 | 19.613094948691 | | 1995 | 17.94657901957 | | 1994 | 18.316896343828 | | 1993 | 16.039902141132 | | 1992 | 11.853542953245 | | 1991 | 13.948431111909 | | 1990 | 16.162197036989 | | 1989 | 18.701755188357 | | 1988 | 21.429332710918 | | 1987 | 16.379794514275 | | 1986 | 16.699493908001 | | 1985 | 13.438259775312 | | 1984 | -2.097173598413 | | 1983 | 4.9952606815175 | | 1982 | 6.7793424716356 | | 1981 | 10.099609030193 | | 1980 | 8.8905512401461 | | 1979 | 20.309619553357 | | 1978 | 16.997961093275 | | 1977 | 15.893997802533 | | 1976 | 18.45969900038 | | 1975 | 22.19011577295 | | 1974 | 15.00874694603 | | 1973 | 19.248840694302 | | 1972 | 19.873611552782 | | 1971 | 9.0740289828983 | | 1970 | 10.641070622686 | | 1969 | 7.2653392929703 | | 1968 | 4.5408210545825 |
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