| Year | Value | | 2024 | 51.869991025547 | | 2023 | 51.879485629413 | | 2022 | 51.887259049082 | | 2021 | 51.880102674505 | | 2020 | 51.852503725733 | | 2019 | 51.828364126232 | | 2018 | 51.822200281263 | | 2017 | 51.819748981234 | | 2016 | 51.818187643372 | | 2015 | 51.823467230444 | | 2014 | 51.833139048681 | | 2013 | 51.847037995714 | | 2012 | 51.863812861066 | | 2011 | 51.884499152821 | | 2010 | 51.915397855995 | | 2009 | 51.960034027051 | | 2008 | 52.013842252052 | | 2007 | 52.071748624044 | | 2006 | 52.130480807609 | | 2005 | 52.193435543828 | | 2004 | 52.263894608783 | | 2003 | 52.272164076274 | | 2002 | 52.197972122203 | | 2001 | 52.103187747441 | | 2000 | 52.00459165492 | | 1999 | 51.908217625256 | | 1998 | 51.817249917204 | | 1997 | 51.733216738512 | | 1996 | 51.657076918213 | | 1995 | 51.588425353987 | | 1994 | 51.526486081432 | | 1993 | 51.469433958659 | | 1992 | 51.418165553816 | | 1991 | 51.377253191353 | | 1990 | 51.354801070755 | | 1989 | 51.34828528968 | | 1988 | 51.351404208715 | | 1987 | 51.364046085517 | | 1986 | 51.38160334215 | | 1985 | 51.402404974987 | | 1984 | 51.42674361264 | | 1983 | 51.456307426858 | | 1982 | 51.493104278935 | | 1981 | 51.537188849787 | | 1980 | 51.589377928251 | | 1979 | 51.645823654473 | | 1978 | 51.698993576465 | | 1977 | 51.756666602123 | | 1976 | 51.826422337949 | | 1975 | 51.90577043884 |
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