| Year | Value | | 2022 | 2.0578763361416 | | 2021 | 2.1444256598866 | | 2020 | 2.1844269687828 | | 2019 | 2.2624249349744 | | 2018 | 2.244592398926 | | 2017 | 2.2686235049359 | | 2016 | 2.3335809310368 | | 2015 | 2.3468160424861 | | 2014 | 2.3051313326852 | | 2013 | 2.2399257437126 | | 2012 | 2.1798818740268 | | 2011 | 2.1165748159372 | | 2010 | 2.030277734453 | | 2009 | 1.9464797567462 | | 2008 | 1.8774868447073 | | 2007 | 1.838824311989 | | 2006 | 1.8342691571004 | | 2005 | 1.8636773634387 | | 2004 | 1.9106251987622 | | 2003 | 1.9787925250997 | | 2002 | 2.0531871821844 | | 2001 | 2.1419215984227 | | 2000 | 1.3389580697221 | | 1999 | 0.53068354315633 | | 1998 | 0.64749139230095 | | 1997 | 1.3407790578684 | | 1996 | 1.9985482504791 | | 1995 | 1.9143658196137 | | 1994 | 1.4960963578712 | | 1993 | 1.0831462353554 | | 1992 | 1.7820204054721 | | 1991 | 2.5232595433676 | | 1990 | 2.8746641592761 | | 1989 | 3.1709800651789 | | 1988 | 3.0606054156872 | | 1987 | 3.0380678362573 | | 1986 | 2.9113962687061 | | 1985 | 2.8531987182611 | | 1984 | 2.8665932592832 | | 1983 | 2.8248009456274 | | 1982 | 2.8003692075441 | | 1981 | 2.7974056588447 | | 1980 | 2.8398452318007 | | 1979 | 2.9565052448608 | | 1978 | 3.0610580981565 | | 1977 | 3.082501195782 | | 1976 | 3.0678250872787 | | 1975 | 3.0430563814664 | | 1974 | 3.0135205585639 | | 1973 | 3.0050815371399 | | 1972 | 3.0163038696237 | | 1971 | 3.0580539146297 | | 1970 | 3.1622186098587 | | 1969 | 3.2698718937982 | | 1968 | 3.3184819976778 | | 1967 | 3.3447224970502 | | 1966 | 3.3837031220071 | | 1965 | 3.4314960994801 | | 1964 | 3.4765430886076 | | 1963 | 3.5092149020212 | | 1962 | 3.5221151183237 | | 1961 | 3.5042149756496 |
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