| Year | Value | | 2021 | 31.258512354496 | | 2020 | 31.549606164852 | | 2019 | 31.870492408381 | | 2018 | 32.181107565799 | | 2017 | 32.50116298867 | | 2016 | 32.915972929543 | | 2015 | 33.3144454497 | | 2014 | 33.859459908236 | | 2013 | 34.402131853705 | | 2012 | 34.825651723652 | | 2011 | 35.262984287306 | | 2010 | 35.638363946153 | | 2009 | 35.972668237738 | | 2008 | 36.203413572858 | | 2007 | 36.270889673373 | | 2006 | 36.145936767485 | | 2005 | 36.38537953698 | | 2004 | 36.738186294305 | | 2003 | 37.040243626525 | | 2002 | 37.499666257422 | | 2001 | 37.671514125017 | | 2000 | 37.867960773178 | | 1999 | 38.151877550645 | | 1998 | 38.323362717603 | | 1997 | 38.775006311022 | | 1996 | 39.37137373249 | | 1995 | 39.930480090512 | | 1994 | 40.332350755944 | | 1993 | 40.796778941937 | | 1992 | 41.426551682076 | | 1991 | 41.916231234898 | | 1990 | 42.259272978934 | | 1989 | 42.637916345517 | | 1988 | 43.015890976205 | | 1987 | 43.468858440997 | | 1986 | 43.795452277905 | | 1985 | 44.026296515139 | | 1984 | 44.340020499471 | | 1983 | 44.68035655928 | | 1982 | 44.772967558363 | | 1981 | 44.779807219353 | | 1980 | 44.578525358856 | | 1979 | 44.67409824843 | | 1978 | 44.845009497537 | | 1977 | 44.846994608117 | | 1976 | 44.859826527966 | | 1975 | 44.94974183758 | | 1974 | 44.948111815374 | | 1973 | 44.913330920423 | | 1972 | 44.948863241507 | | 1971 | 45.018303890473 | | 1970 | 45.00561968796 | | 1969 | 45.11058031179 | | 1968 | 45.361858837613 | | 1967 | 45.657618749687 | | 1966 | 45.912365893985 | | 1965 | 46.176800840013 | | 1964 | 46.350733332734 | | 1963 | 46.521087339029 | | 1962 | 46.708876422549 | | 1961 | 46.680554447413 | | 1960 | 46.61344630097 |
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