| Year | Value | | 2021 | 1.3969448271986 | | 2020 | 1.467538716553 | | 2019 | 1.6015910073601 | | 2018 | 1.6469679006149 | | 2017 | 1.8058217055366 | | 2016 | 1.7907219743525 | | 2015 | 1.7400111337559 | | 2014 | 1.8001847658294 | | 2013 | 1.76128308139 | | 2012 | 1.8157432558303 | | 2011 | 1.730441176113 | | 2010 | 1.7430094899383 | | 2009 | 1.7619711615914 | | 2008 | 1.7542772314357 | | 2007 | 1.7251028838881 | | 2006 | 1.7063518008623 | | 2005 | 1.6990481272877 | | 2004 | 1.6968018699618 | | 2003 | 1.6753425195782 | | 2002 | 1.6810507137918 | | 2001 | 1.686064748878 | | 2000 | 1.7375023496439 | | 1999 | 1.6858178910009 | | 1998 | 1.7049919129528 | | 1997 | 1.7288249380075 | | 1996 | 1.7690056425359 | | 1995 | 1.8181043805387 | | 1994 | 1.8729150692999 | | 1993 | 1.9338224714963 | | 1992 | 2.0220135705802 | | 1991 | 2.1575051425921 | | 1990 | 2.5536032146454 | | 1989 | 2.5888458686642 | | 1988 | 2.6327950592858 | | 1987 | 2.8042972184288 | | 1986 | 2.7989850554369 | | 1985 | 2.7608482649115 | | 1984 | 2.7727803437035 | | 1983 | 2.7710281875748 | | 1982 | 3.0432734605652 | | 1981 | 2.9442452413223 | | 1980 | 2.9306873665297 | | 1979 | 2.9559175662945 | | 1978 | 2.9584981344992 | | 1977 | 3.0606391498165 | | 1976 | 3.3185789454298 | | 1975 | 3.5418910125987 | | 1974 | 3.9240475687636 | | 1973 | 4.2837602343672 | | 1972 | 4.5397059063183 | | 1971 | 4.7939290614094 | | 1970 | 5.1371111497306 | | 1969 | 5.2069462846025 | | 1968 | 5.4276667107855 | | 1967 | 5.0440007550128 | | 1966 | 5.3260639528295 | | 1965 | 5.5240920165935 | | 1964 | 5.5882310910223 | | 1963 | 6.106401546836 | | 1962 | 5.2896241728555 | | 1961 | 3.9988777689889 | | 1960 | 4.378530007965 |
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