| Year | Value | | 2023 | 51.256876405148 | | 2022 | 51.202154519985 | | 2021 | 51.146748355285 | | 2020 | 51.089238339748 | | 2019 | 51.031290659101 | | 2018 | 50.976238845738 | | 2017 | 50.925195011081 | | 2016 | 50.876940296832 | | 2015 | 50.830487521441 | | 2014 | 50.785746291887 | | 2013 | 50.743389856255 | | 2012 | 50.703111226094 | | 2011 | 50.665244998754 | | 2010 | 50.631668114792 | | 2009 | 50.609043485511 | | 2008 | 50.595137550627 | | 2007 | 50.581366739315 | | 2006 | 50.566466279653 | | 2005 | 50.55019342698 | | 2004 | 50.532753399336 | | 2003 | 50.514182873615 | | 2002 | 50.494238540095 | | 2001 | 50.473073216394 | | 2000 | 50.446958848628 | | 1999 | 50.400754143194 | | 1998 | 50.337049960315 | | 1997 | 50.272785730405 | | 1996 | 50.210568036285 | | 1995 | 50.151408928781 | | 1994 | 50.097229951349 | | 1993 | 50.046703259233 | | 1992 | 49.991979314486 | | 1991 | 49.934096529326 | | 1990 | 49.874849120805 | | 1989 | 49.835498200476 | | 1988 | 49.820769820875 | | 1987 | 49.807754740907 | | 1986 | 49.796064053825 | | 1985 | 49.784055348923 | | 1984 | 49.770060643089 | | 1983 | 49.753125085961 | | 1982 | 49.73304636443 | | 1981 | 49.710313429344 | | 1980 | 49.685937712931 | | 1979 | 49.661988394719 | | 1978 | 49.640343268132 | | 1977 | 49.621852314124 | | 1976 | 49.606738070556 | | 1975 | 49.594510308438 | | 1974 | 49.584576995942 |
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