| Year | Value | | 2021 | 1.5090153723544 | | 2020 | 1.4811308862756 | | 2019 | 1.5045661337261 | | 2018 | 1.5234371939272 | | 2017 | 1.5423804720464 | | 2016 | 1.5677245160818 | | 2015 | 1.5481761252671 | | 2014 | 1.5502357253707 | | 2013 | 1.5294017682455 | | 2012 | 1.5539528920888 | | 2011 | 1.5599833679612 | | 2010 | 1.5789503691464 | | 2009 | 1.5637225540978 | | 2008 | 1.579817538834 | | 2007 | 1.5399320515301 | | 2006 | 1.523126843626 | | 2005 | 1.5002843742674 | | 2004 | 1.4954628274929 | | 2003 | 1.4728682304283 | | 2002 | 1.4587382016727 | | 2001 | 1.4565597323372 | | 2000 | 1.4694284380696 | | 1999 | 1.4302466915924 | | 1998 | 1.417809647766 | | 1997 | 1.4203007392508 | | 1996 | 1.3998537024222 | | 1995 | 1.3857457730389 | | 1994 | 1.3988346036548 | | 1993 | 1.436274959927 | | 1992 | 1.4682962577157 | | 1991 | 1.4895232740943 | | 1990 | 1.535773547606 | | 1989 | 1.5311559503656 | | 1988 | 1.5661584535573 | | 1987 | 1.5594072682528 | | 1986 | 1.5826306175466 | | 1985 | 1.5943119287882 | | 1984 | 1.6059100621447 | | 1983 | 1.6426460612664 | | 1982 | 1.7006759625674 | | 1981 | 1.7273241661337 | | 1980 | 1.775719720422 | | 1979 | 1.8018456265786 | | 1978 | 1.852400288368 | | 1977 | 1.9069118730717 | | 1976 | 1.9809966709024 | | 1975 | 2.0205810995821 | | 1974 | 2.096115017098 | | 1973 | 2.1288087124597 | | 1972 | 2.2320003536119 | | 1971 | 2.3380133348091 | | 1970 | 2.384556441076 | | 1969 | 2.4794984464699 | | 1968 | 2.5426907835258 | | 1967 | 2.6120953107398 | | 1966 | 2.6568541670878 | | 1965 | 2.6749083891165 | | 1964 | 2.7265171499222 | | 1963 | 2.6735936149464 | | 1962 | 2.635350390774 | | 1961 | 2.6301450485443 | | 1960 | 2.599772382888 |
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