| Year | Value | | 2021 | 1.5676413556164 | | 2020 | 1.6314075348747 | | 2019 | 1.7471082001492 | | 2018 | 1.7923975848891 | | 2017 | 1.9323175141411 | | 2016 | 1.9295597266997 | | 2015 | 1.8975705447847 | | 2014 | 1.9583117716349 | | 2013 | 1.9311038307601 | | 2012 | 1.9832358471823 | | 2011 | 1.9129399848323 | | 2010 | 1.9173338835367 | | 2009 | 1.9336197052647 | | 2008 | 1.9320647352524 | | 2007 | 1.9090226760966 | | 2006 | 1.8935382149155 | | 2005 | 1.8864601535712 | | 2004 | 1.8821194516648 | | 2003 | 1.8650927263304 | | 2002 | 1.8756810368249 | | 2001 | 1.892857640503 | | 2000 | 1.9427040706869 | | 1999 | 1.9086933142651 | | 1998 | 1.9317866381407 | | 1997 | 1.9596038151141 | | 1996 | 2.0006377219103 | | 1995 | 2.0442631179463 | | 1994 | 2.0905730622897 | | 1993 | 2.1441573649952 | | 1992 | 2.2217379833431 | | 1991 | 2.350536355002 | | 1990 | 2.7051569917408 | | 1989 | 2.7481797186875 | | 1988 | 2.7992863654432 | | 1987 | 2.9617713039522 | | 1986 | 2.9676419827923 | | 1985 | 2.9496278261825 | | 1984 | 2.9808908324469 | | 1983 | 2.994642682707 | | 1982 | 3.2519467025564 | | 1981 | 3.173715746864 | | 1980 | 3.178899614167 | | 1979 | 3.2193419857663 | | 1978 | 3.2416697703762 | | 1977 | 3.3483290785055 | | 1976 | 3.5943663510139 | | 1975 | 3.8080390995691 | | 1974 | 4.1620555713284 | | 1973 | 4.4995533047652 | | 1972 | 4.7376833047626 | | 1971 | 4.9824232611575 | | 1970 | 5.3052575432477 | | 1969 | 5.3752664519692 | | 1968 | 5.5715655421846 | | 1967 | 5.2266533831124 | | 1966 | 5.518069425459 | | 1965 | 5.6934656120217 | | 1964 | 5.7494800547464 | | 1963 | 6.2105346663115 | | 1962 | 5.4746303907596 | | 1961 | 4.3114165570615 | | 1960 | 4.6144112492031 |
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