| Year | Value | | 2024 | 17.299743423337 | | 2023 | 17.474943559165 | | 2022 | 17.649910264603 | | 2021 | 17.822072092072 | | 2020 | 18.049011128943 | | 2019 | 18.263632795206 | | 2018 | 18.449965835335 | | 2017 | 18.598105416777 | | 2016 | 18.69769889063 | | 2015 | 18.75343297233 | | 2014 | 18.788816838444 | | 2013 | 18.816560042542 | | 2012 | 18.848651675121 | | 2011 | 18.896857456579 | | 2010 | 18.987910774706 | | 2009 | 19.140816110608 | | 2008 | 19.33710550834 | | 2007 | 19.557438486322 | | 2006 | 19.780933900721 | | 2005 | 20.023938411704 | | 2004 | 20.307693665169 | | 2003 | 20.608619325761 | | 2002 | 20.900976519777 | | 2001 | 21.158523505026 | | 2000 | 21.370700449105 | | 1999 | 21.556069067064 | | 1998 | 21.730617691818 | | 1997 | 21.909660120913 | | 1996 | 22.107536607031 | | 1995 | 22.36435174125 | | 1994 | 22.682871457906 | | 1993 | 23.003431374409 | | 1992 | 23.264716757194 | | 1991 | 23.404615868484 | | 1990 | 23.443556292298 | | 1989 | 23.465923709505 | | 1988 | 23.482299454337 | | 1987 | 23.502057125081 | | 1986 | 23.534218093873 | | 1985 | 23.621527489022 | | 1984 | 23.781952367862 | | 1983 | 23.971706522019 | | 1982 | 24.14639446455 | | 1981 | 24.260534519992 | | 1980 | 24.323286486719 | | 1979 | 24.378629525107 | | 1978 | 24.423799661254 | | 1977 | 24.454660434704 | | 1976 | 24.467045933521 |
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