| Year | Value | | 2023 | 9.9 | | 2022 | 9.9 | | 2021 | 14.1 | | 2020 | 15.1 | | 2019 | 15.2 | | 2018 | 17.8 | | 2017 | 17.2 | | 2016 | 17.9 | | 2015 | 16.9 | | 2014 | 20.9 | | 2013 | 21.8 | | 2012 | 24.9 | | 2011 | 25.5 | | 2010 | 30 | | 2009 | 32.2 | | 2008 | 35.2 | | 2007 | 37.4 | | 2006 | 39.5 | | 2004 | 46.7 | | 2002 | 53.3 | | 2000 | 61.2 | | 1999 | 60.6 | | 1998 | 56.3 | | 1996 | 58.6 | | 1994 | 62.5 | | 1992 | 68 | | 1990 | 74 | | 1988 | 77.9 | | 1981 | 78.5 |
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