| Year | Value | | 2022 | 2.3885118883062 | | 2021 | 2.2219952529547 | | 2020 | 2.2291407862181 | | 2019 | 1.9750953903269 | | 2018 | 2.0448002497994 | | 2017 | 1.9016599808149 | | 2016 | 1.9492177066218 | | 2015 | 2.1405458303192 | | 2014 | 1.9190367598286 | | 2013 | 1.7984499357718 | | 2012 | 1.8146767703888 | | 2011 | 1.803183618352 | | 2010 | 1.8478521550955 | | 2009 | 1.7974162278384 | | 2008 | 1.7521397106811 | | 2007 | 2.0020328284104 | | 2006 | 1.9207503803424 | | 2005 | 1.9261298699201 | | 2004 | 1.8732410064926 | | 2003 | 1.9080420365751 | | 2002 | 1.9006386493251 | | 2001 | 1.905638001218 | | 2000 | 1.8303365853136 | | 1999 | 1.9011357712239 | | 1998 | 2.019081314424 | | 1997 | 2.031306696575 | | 1996 | 1.9679194008086 | | 1995 | 1.9555070546999 | | 1994 | 2.2732321033505 | | 1993 | 2.4691873154449 | | 1992 | 2.2309994432071 | | 1991 | 2.2516502252021 | | 1990 | 2.6124857435063 | | 1989 | 1.8171215104928 | | 1988 | 2.5920550811705 | | 1987 | 2.7626918536009 | | 1986 | 2.9414035358604 | | 1985 | 3.0157970320728 | | 1984 | 3.0550373134328 | | 1983 | 2.9029462738302 | | 1982 | 3.245582401731 | | 1981 | 2.913185615692 | | 1980 | 2.6523297491039 |
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