| Year | Value | | 2021 | 3.1819988517423 | | 2020 | 3.1312105286402 | | 2019 | 3.1002958971868 | | 2018 | 3.1025040851477 | | 2017 | 3.1025040851477 | | 2016 | 2.9324736121539 | | 2015 | 2.9192244843881 | | 2014 | 2.8816852890518 | | 2013 | 2.8397297177936 | | 2012 | 2.8397297177936 | | 2011 | 2.8397297177936 | | 2010 | 2.7977741465354 | | 2009 | 2.7977741465354 | | 2008 | 2.7491940113943 | | 2007 | 2.6763238086826 | | 2006 | 2.5416243430641 | | 2005 | 2.5018769597668 | | 2004 | 2.4797950801572 | | 2003 | 2.4466722607428 | | 2002 | 2.435631320938 | | 2001 | 2.4135494413284 | | 2000 | 2.4025085015236 | | 1999 | 2.3870511857969 | | 1998 | 2.3495119904606 | | 1997 | 2.2589762840613 | | 1996 | 2.2435189683346 | | 1995 | 2.1949388331935 | | 1994 | 2.1596078258181 | | 1993 | 2.1441505100914 | | 1992 | 2.0801130592236 | | 1991 | 2.060239367575 | | 1990 | 2.0491984277702 | | 1989 | 2.0006182926291 | | 1988 | 1.9586627213708 | | 1987 | 1.9034580223469 | | 1986 | 1.8902088945811 | | 1985 | 1.8283796316742 | | 1984 | 1.7820076844941 | | 1983 | 1.7488848650797 | | 1982 | 1.7555094289626 | | 1981 | 1.7731749326503 | | 1980 | 1.7599258048845 | | 1979 | 1.7510930530407 | | 1978 | 1.7290111734311 | | 1977 | 1.6738064744071 | | 1976 | 1.6451000309146 | | 1975 | 1.6031444596564 | | 1974 | 1.5545643245153 | | 1973 | 1.4949432495694 | | 1972 | 1.4529876783112 | | 1971 | 1.4309057987016 | | 1970 | 1.3779092876386 | | 1969 | 1.3403700923023 | | 1968 | 1.2962063330831 | | 1967 | 1.2542507618248 | | 1966 | 1.2078788146447 | | 1965 | 1.2145033785276 | | 1964 | 1.2145033785276 | | 1963 | 1.1504659276598 | | 1962 | 1.1416331758159 | | 1961 | 1.1195512962063 |
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