| Year | Value | | 2021 | 60.016120887181 | | 2020 | 60.5506923913 | | 2019 | 61.171508576006 | | 2018 | 60.819345432878 | | 2017 | 60.494183404286 | | 2016 | 60.11564775989 | | 2015 | 59.623281133145 | | 2014 | 59.212615187968 | | 2013 | 58.737170938116 | | 2012 | 58.277382360413 | | 2011 | 57.743538647692 | | 2010 | 56.944840892646 | | 2009 | 56.621767365235 | | 2008 | 55.957762660072 | | 2007 | 55.386755447965 | | 2006 | 54.795720766323 | | 2005 | 53.975561923095 | | 2004 | 53.361247073717 | | 2003 | 52.854181087391 | | 2002 | 52.250053449779 | | 2001 | 51.746573623149 | | 2000 | 51.247522644497 | | 1999 | 50.13628560482 | | 1998 | 49.181506690449 | | 1997 | 49.519968156211 | | 1996 | 49.156586796228 | | 1995 | 49.032445642048 | | 1994 | 48.599660516254 | | 1993 | 48.082182320351 | | 1992 | 47.424277709667 | | 1991 | 47.449729737212 | | 1990 | 47.317792430189 | | 1989 | 47.332536838534 | | 1988 | 46.120814739579 | | 1987 | 46.288661145945 | | 1986 | 45.691467422621 | | 1985 | 44.863608401847 | | 1984 | 44.459779227646 | | 1983 | 44.747924751535 | | 1982 | 45.968342550525 | | 1981 | 45.793980933622 | | 1980 | 45.513765693083 | | 1979 | 45.090687401363 | | 1978 | 44.710046642578 | | 1977 | 44.614893293894 | | 1976 | 44.267607304742 | | 1975 | 43.657697118045 | | 1974 | 43.297997353788 | | 1973 | 43.237892043986 | | 1972 | 42.457051671532 | | 1971 | 42.632371667929 | | 1970 | 42.324013444191 | | 1969 | 42.008791692699 | | 1968 | 41.776066881241 | | 1967 | 41.44075740039 | | 1966 | 40.835085330542 | | 1965 | 40.847468839535 | | 1964 | 40.712930489749 | | 1963 | 40.261733760427 | | 1962 | 40.196949713026 | | 1961 | 39.510568252006 | | 1960 | 39.215905859382 |
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