| Year | Value | | 2022 | 1.8229447042006 | | 2021 | 1.7320783356475 | | 2020 | 1.8968786889683 | | 2019 | 1.806655292324 | | 2018 | 1.790997623533 | | 2017 | 1.8620850683037 | | 2016 | 1.9334346220471 | | 2015 | 1.9224842506618 | | 2014 | 1.9241524504381 | | 2013 | 1.9044795734531 | | 2012 | 1.875817882453 | | 2011 | 1.822146842696 | | 2010 | 1.9033074441839 | | 2009 | 2.0527062361104 | | 2008 | 1.8484934484709 | | 2007 | 1.8288888399384 | | 2006 | 1.858061730425 | | 2005 | 1.9032394109475 | | 2004 | 1.9257571238824 | | 2003 | 1.9817631737864 | | 2002 | 2.0261543246765 | | 2001 | 1.9733169313468 | | 2000 | 1.8991728177778 | | 1999 | 1.9708412357255 | | 1998 | 1.8540349019465 | | 1997 | 1.9901249138523 | | 1996 | 2.0268902622062 | | 1995 | 2.11409457667 | | 1994 | 2.1999871260874 | | 1993 | 2.2268996167788 | | 1992 | 2.3947232379593 | | 1991 | 2.2164374343819 | | 1990 | 2.4764167549311 | | 1989 | 2.7365452370692 | | 1987 | 3.3567347614454 | | 1986 | 3.3979687905901 | | 1985 | 3.2118534048896 | | 1984 | 3.1277097665512 | | 1983 | 3.3533919837663 | | 1982 | 3.3612795689114 | | 1981 | 3.2758666843366 | | 1980 | 3.0152387629882 | | 1979 | 3.307641963187 | | 1978 | 4.0869972165956 | | 1977 | 4.4328100929445 | | 1976 | 4.5397510615848 | | 1975 | 4.4416102545468 | | 1974 | 3.6443872990643 | | 1973 | 2.952584528729 | | 1969 | 3.3754519316607 | | 1968 | 3.2705471815603 | | 1965 | 3.1413144167163 | | 1964 | 2.9095022810491 | | 1963 | 2.9062170858589 |
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