| Year | Value | | 2023 | 49.933875136113 | | 2022 | 49.957084015901 | | 2021 | 49.989854792778 | | 2020 | 50.026001347493 | | 2019 | 50.055990866949 | | 2018 | 50.076497375473 | | 2017 | 50.091480682345 | | 2016 | 50.107267312013 | | 2015 | 50.123191843411 | | 2014 | 50.138009107932 | | 2013 | 50.152982805093 | | 2012 | 50.170266315372 | | 2011 | 50.19048994584 | | 2010 | 50.187484059455 | | 2009 | 50.159754699272 | | 2008 | 50.132288157881 | | 2007 | 50.127928973932 | | 2006 | 50.14718964424 | | 2005 | 50.166299201941 | | 2004 | 50.184909842411 | | 2003 | 50.203274340833 | | 2002 | 50.221374594109 | | 2001 | 50.239120981093 | | 2000 | 50.256266770674 | | 1999 | 50.273026401041 | | 1998 | 50.291422763126 | | 1997 | 50.312642365994 | | 1996 | 50.336461960487 | | 1995 | 50.362429006591 | | 1994 | 50.384655586304 | | 1993 | 50.402662464709 | | 1992 | 50.421585622571 | | 1991 | 50.440784069612 | | 1990 | 50.460197473109 | | 1989 | 50.480339660848 | | 1988 | 50.501602441093 | | 1987 | 50.524264316318 | | 1986 | 50.548596413148 | | 1985 | 50.575445061334 | | 1984 | 50.604836359008 | | 1983 | 50.636096942311 | | 1982 | 50.669010941527 | | 1981 | 50.703832291359 | | 1980 | 50.742263352561 | | 1979 | 50.82393836285 | | 1978 | 50.949407938622 | | 1977 | 51.08070694011 | | 1976 | 51.219120961499 | | 1975 | 51.248692943658 |
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