| Year | Value | | 2022 | 3.7129878184295 | | 2021 | 3.7074237846131 | | 2020 | 3.7271007252322 | | 2019 | 3.7654448854226 | | 2018 | 3.78759649907 | | 2017 | 3.8269339822334 | | 2016 | 3.8670912054247 | | 2015 | 3.8288593838123 | | 2014 | 3.781178770258 | | 2013 | 3.8182059410019 | | 2012 | 3.810576036689 | | 2011 | 3.7467201784057 | | 2010 | 3.7307031511636 | | 2009 | 3.7011707741736 | | 2008 | 3.6725447632779 | | 2007 | 3.6412063628467 | | 2006 | 3.6144258347131 | | 2005 | 3.5881749432371 | | 2004 | 3.5491829873009 | | 2003 | 3.5044610030773 | | 2002 | 3.4721547356716 | | 2001 | 3.4565321331852 | | 2000 | 3.423750055835 | | 1999 | 3.4101137097437 | | 1998 | 3.3795614544203 | | 1997 | 3.3355320311279 | | 1996 | 3.3665916349815 | | 1995 | 3.3212767493053 | | 1994 | 3.213800516686 | | 1993 | 3.1541025042952 | | 1992 | 3.1127764583029 | | 1991 | 3.1050719076775 | | 1990 | 3.0884637933207 | | 1989 | 3.0502491493597 | | 1988 | 3.0392872021459 | | 1987 | 3.034166720538 | | 1986 | 3.0249808825846 | | 1985 | 3.0192527210682 | | 1984 | 3.0167664347127 | | 1983 | 3.0331666864235 | | 1982 | 3.065548080295 | | 1981 | 3.079886789983 | | 1980 | 3.0597943317177 | | 1979 | 2.9974690818089 | | 1978 | 2.89233988342 | | 1977 | 2.8033802090137 | | 1976 | 2.7552327416637 | | 1975 | 2.7130929307991 | | 1974 | 2.6756183017942 | | 1973 | 2.6652488418057 | | 1972 | 2.6586955089128 | | 1971 | 2.6908354986495 | | 1970 | 2.7510497015301 | | 1969 | 2.7909352951418 | | 1968 | 2.8392089792491 | | 1967 | 2.8768986981826 | | 1966 | 2.892898998102 | | 1965 | 2.9165582273674 | | 1964 | 2.9464849518246 | | 1963 | 2.963648863286 | | 1962 | 2.9683329405315 | | 1961 | 2.9607677421699 |
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