| Year | Value | | 2022 | 2.7439595634118 | | 2021 | 3.0455693242642 | | 2020 | 3.3347876872666 | | 2019 | 3.0812224455401 | | 2018 | 2.9247594454388 | | 2017 | 3.2485791949909 | | 2016 | 3.410662582733 | | 2015 | 2.8251944990366 | | 2014 | 2.2119971444901 | | 2013 | 2.1471710497407 | | 2012 | 2.3437721258219 | | 2011 | 2.4044300323858 | | 2010 | 2.7590798803343 | | 2009 | 3.2628207375053 | | 2008 | 3.0976203987662 | | 2007 | 2.9000146895276 | | 2006 | 2.7545460951655 | | 2005 | 2.8580041850403 | | 2004 | 3.5058740413487 | | 2003 | 4.0073665887052 | | 2002 | 4.1102064895166 | | 2001 | 3.8312230326037 | | 2000 | 3.1912429508106 | | 1999 | 3.0920737553024 | | 1998 | 3.7793988444373 | | 1997 | 3.1972046271841 | | 1996 | 2.8995214983348 | | 1995 | 3.5064797194978 | | 1994 | 4.0081203955955 | | 1993 | 4.9334919729801 | | 1992 | 4.4828090491255 | | 1991 | 4.6034699272968 | | 1990 | 4.4451580347722 | | 1989 | 3.5690100801241 | | 1988 | 4.8996770228942 | | 1987 | 4.7944145747109 | | 1986 | 2.6635958026935 | | 1985 | 2.1847314885705 | | 1984 | 2.4372708318355 | | 1983 | 2.3263376983783 | | 1982 | 2.6947438604289 | | 1981 | 2.9782981153627 | | 1980 | 3.2076205287714 | | 1979 | 3.5014993826072 | | 1978 | 2.8749090026692 | | 1977 | 1.3565725691474 |
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