| Year | Value | | 2022 | 48.706343208221 | | 2021 | 48.689243091671 | | 2020 | 48.662893541385 | | 2019 | 48.610642963548 | | 2018 | 48.555228537058 | | 2017 | 48.495718630582 | | 2016 | 48.43765648117 | | 2015 | 48.382033061367 | | 2014 | 48.324452167288 | | 2013 | 48.271164444189 | | 2012 | 48.222975601366 | | 2011 | 48.177139703142 | | 2010 | 48.132113987697 | | 2009 | 48.090996844001 | | 2008 | 48.058567974247 | | 2007 | 48.031338780424 | | 2006 | 48.005776944739 | | 2005 | 47.985035491269 | | 2004 | 47.969640023856 | | 2003 | 47.958918114858 | | 2002 | 47.95445683433 | | 2001 | 47.956236399595 | | 2000 | 47.998541915745 | | 1999 | 48.077346917017 | | 1998 | 48.149286298598 | | 1997 | 48.211020439265 | | 1996 | 48.264709912185 | | 1995 | 48.298620790081 | | 1994 | 48.300544504677 | | 1993 | 48.285797333822 | | 1992 | 48.288995254799 | | 1991 | 48.330836354959 | | 1990 | 48.365320278794 | | 1989 | 48.369408446381 | | 1988 | 48.369753308173 | | 1987 | 48.366667615128 | | 1986 | 48.359449489758 | | 1985 | 48.351943705662 | | 1984 | 48.345267800054 | | 1983 | 48.338892524869 | | 1982 | 48.327509000841 | | 1981 | 48.317020905701 | | 1980 | 48.31631248953 | | 1979 | 48.322829524831 | | 1978 | 48.332478747342 | | 1977 | 48.339449738059 | | 1976 | 48.340780272127 | | 1975 | 48.335534481599 | | 1974 | 48.325781956092 | | 1973 | 48.312362336226 | | 1972 | 48.292670641252 | | 1971 | 48.261815923846 | | 1970 | 48.219531598139 | | 1969 | 48.168882772787 | | 1968 | 48.108647660683 | | 1967 | 48.040961659451 | | 1966 | 47.963991798763 | | 1965 | 47.880086407476 | | 1964 | 47.796255416037 | | 1963 | 47.713592212741 | | 1962 | 47.636687505096 | | 1961 | 47.568888934925 | | 1960 | 47.513002559234 |
|