| Year | Value | | 2021 | 1.5204420610728 | | 2020 | 1.5972662617895 | | 2019 | 1.7377842395825 | | 2018 | 1.7775947219828 | | 2017 | 1.9526445018353 | | 2016 | 1.9338907714553 | | 2015 | 1.8741633227356 | | 2014 | 1.9489560387198 | | 2013 | 1.9182012768322 | | 2012 | 1.9823278179994 | | 2011 | 1.8932651504407 | | 2010 | 1.9056392589102 | | 2009 | 1.9326092324082 | | 2008 | 1.9291731595723 | | 2007 | 1.9066302477146 | | 2006 | 1.8868144140874 | | 2005 | 1.8744176528868 | | 2004 | 1.8654324464957 | | 2003 | 1.8471935209061 | | 2002 | 1.8536696813612 | | 2001 | 1.8605105981723 | | 2000 | 1.9131261343087 | | 1999 | 1.8563355902647 | | 1998 | 1.8704694788292 | | 1997 | 1.8956771291145 | | 1996 | 1.9315879016085 | | 1995 | 1.9729211031246 | | 1994 | 2.0210380154391 | | 1993 | 2.0858427690549 | | 1992 | 2.1659391054329 | | 1991 | 2.3023647567317 | | 1990 | 2.7338358562204 | | 1989 | 2.7603967676063 | | 1988 | 2.7995542059932 | | 1987 | 2.9849833866881 | | 1986 | 2.9863154099388 | | 1985 | 2.9620607552235 | | 1984 | 2.989434719194 | | 1983 | 2.9911425400555 | | 1982 | 3.3226769728562 | | 1981 | 3.2203297931783 | | 1980 | 3.21441172711 | | 1979 | 3.2460114232356 | | 1978 | 3.2512582333287 | | 1977 | 3.3720628457607 | | 1976 | 3.6896662244063 | | 1975 | 3.968740162167 | | 1974 | 4.4324325250796 | | 1973 | 4.8836591303581 | | 1972 | 5.1997620725487 | | 1971 | 5.5167822285409 | | 1970 | 5.9461371231628 | | 1969 | 6.0278147036347 | | 1968 | 6.2844663830857 | | 1967 | 5.7880776616636 | | 1966 | 6.1585079070444 | | 1965 | 6.3866122451732 | | 1964 | 6.445199688111 | | 1963 | 7.0613295686037 | | 1962 | 6.0265757591694 | | 1961 | 4.3874873380693 | | 1960 | 4.8419815606764 |
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