| Year | Value | | 2021 | 12.858628582744 | | 2020 | 11.640616892094 | | 2019 | 9.9441978875259 | | 2018 | 10.072220444235 | | 2017 | 10.041576193974 | | 2016 | 10.202993111799 | | 2015 | 10.310722199374 | | 2014 | 10.276605790754 | | 2013 | 10.253823110421 | | 2012 | 10.440091592492 | | 2011 | 10.55206628869 | | 2010 | 10.865673971863 | | 2009 | 10.860398449995 | | 2008 | 11.157635890291 | | 2007 | 11.238203289026 | | 2006 | 11.411839486087 | | 2005 | 11.838230171497 | | 2004 | 11.652802313173 | | 2003 | 11.92128660916 | | 2002 | 11.843451621129 | | 2001 | 11.560852471564 | | 2000 | 11.533007842241 | | 1999 | 11.368783965757 | | 1998 | 10.976318218924 | | 1997 | 11.220578663856 | | 1996 | 11.48954441965 | | 1995 | 11.808646421596 | | 1994 | 11.957856841462 | | 1993 | 11.612327555171 | | 1992 | 10.667445320897 | | 1991 | 10.161237737626 | | 1990 | 9.9230288064122 | | 1989 | 9.7101061010159 | | 1988 | 9.8555565025209 | | 1987 | 9.7727896074051 | | 1986 | 9.6959438750701 | | 1985 | 10.291543996602 | | 1984 | 10.382008865294 | | 1983 | 10.132250766346 | | 1982 | 9.9235044564095 | | 1981 | 10.024907164127 | | 1980 | 10.265820648287 | | 1979 | 10.064172820796 | | 1978 | 10.096643294331 | | 1977 | 9.9911700051834 | | 1976 | 9.9294349562637 | | 1975 | 9.8292621027809 | | 1974 | 9.6160080771945 | | 1973 | 9.6594140174088 | | 1972 | 9.5857458549866 | | 1971 | 9.5941059692687 | | 1970 | 9.5600609917148 | | 1969 | 9.6153601436574 | | 1968 | 9.2997072637722 | | 1967 | 9.2591764520784 | | 1966 | 9.0502582104225 | | 1965 | 9.095874948556 | | 1964 | 9.0045000142225 | | 1963 | 9.2058024504489 | | 1962 | 9.5228089786886 | | 1961 | 9.3711911853177 | | 1960 | 9.475376692509 |
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