| Year | Value | | 2023 | 14.822537357753 | | 2022 | 15.445073409489 | | 2021 | 14.74876072714 | | 2020 | 9.9419451874644 | | 2019 | 10.353979282556 | | 2018 | 12.2080170663 | | 2017 | 11.37715079114 | | 2016 | 8.2120499446856 | | 2015 | 13.476640381956 | | 2014 | 9.8196887881681 | | 2012 | 12.03099703307 | | 2011 | 10.800973105397 | | 2010 | 13.843109445777 | | 2009 | 13.873346427893 | | 2008 | 11.807050580174 | | 2007 | 12.792614498068 | | 2006 | 10.458374884281 | | 2005 | 8.5596286343695 | | 2004 | 6.8405101044108 | | 2003 | 4.4252977719676 | | 1995 | 9.6375203567509 | | 1994 | 13.809344670911 | | 1993 | 12.352032938891 | | 1992 | 10.658649010023 | | 1991 | 9.4898991079395 | | 1990 | 10.206890663434 | | 1989 | 13.130236007399 | | 1988 | 11.649347352349 | | 1987 | 11.421065096913 | | 1986 | 10.025584205364 | | 1985 | 15.293140844665 | | 1984 | 15.02726927883 | | 1983 | 9.0623544260711 | | 1982 | 9.4321332619431 | | 1981 | 12.450543730727 | | 1980 | 5.612753528335 |
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