| Year | Value | | 2022 | 2.3696474066313 | | 2021 | 3.2363674820972 | | 2020 | 3.6342917686241 | | 2019 | 3.0785872494828 | | 2018 | 2.6366124853302 | | 2017 | 2.8651285095717 | | 2016 | 3.5393846390412 | | 2015 | 3.2792904608375 | | 2014 | 3.0868609578118 | | 2013 | 2.2775075950492 | | 2012 | 2.1590344889605 | | 2011 | 2.2422414199017 | | 2010 | 2.8504879006009 | | 2009 | 2.9410720317604 | | 2008 | 2.2617923688784 | | 2007 | 2.3801522443409 | | 2006 | 2.3039631946555 | | 2005 | 2.3494276381516 | | 2004 | 2.532730087631 | | 2003 | 3.7114196180039 | | 2002 | 3.8707827582911 | | 2001 | 3.8863976083707 | | 2000 | 4.0692054900445 | | 1999 | 6.1304183520652 | | 1998 | 7.5298438934803 | | 1997 | 7.1839645521165 | | 1996 | 6.2494938314931 | | 1995 | 5.477265965216 | | 1994 | 5.9824713589184 | | 1993 | 5.7402317352812 | | 1992 | 6.2451447807345 | | 1991 | 6.404350124613 | | 1990 | 6.4377352692633 | | 1989 | 6.2104362703165 | | 1988 | 6.6299770998006 | | 1987 | 3.792514954576 | | 1986 | 4.6707298076174 | | 1985 | 2.8877881402938 | | 1984 | 2.5740844023053 |
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