| Year | Value | | 2021 | 22.098910618489 | | 2020 | 19.169773796934 | | 2019 | 17.923262958664 | | 2018 | 18.440662512442 | | 2017 | 17.767875045735 | | 2016 | 18.477858075122 | | 2015 | 19.814709931637 | | 2014 | 20.522982794948 | | 2013 | 20.958621221766 | | 2012 | 19.320512650964 | | 2011 | 19.390285374221 | | 2010 | 20.99344106459 | | 2009 | 22.011630797778 | | 2008 | 24.091836806591 | | 2007 | 22.394825009455 | | 2006 | 25.419601008202 | | 2005 | 27.517521550768 | | 2004 | 31.043115438109 | | 2003 | 31.380541903731 | | 2002 | 29.78058020391 | | 2001 | 30.940434838544 | | 2000 | 31.192688223938 | | 1999 | 31.678700361011 | | 1998 | 31.841791389327 | | 1997 | 33.16987454538 | | 1996 | 33.066113798664 | | 1995 | 33.949426221184 | | 1994 | 31.346578366446 | | 1993 | 32.537849627919 | | 1992 | 32.764367235633 | | 1991 | 36.188384577843 | | 1990 | 36.966706745719 |
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