| Year | Value | | 2021 | 24.350926443882 | | 2020 | 24.373218836704 | | 2019 | 24.045482777918 | | 2018 | 23.056330403482 | | 2017 | 23.08554134277 | | 2016 | 21.905110138253 | | 2015 | 21.160624224647 | | 2014 | 20.636428613973 | | 2013 | 19.69209478805 | | 2012 | 19.407748642349 | | 2011 | 20.229442507907 | | 2010 | 20.986190269827 | | 2009 | 20.944334727819 | | 2008 | 21.143915772298 | | 2007 | 21.595200025841 | | 2006 | 21.600010948668 | | 2005 | 21.234817556541 | | 2004 | 20.170002607286 | | 2003 | 20.353737251614 | | 2002 | 21.158086511273 | | 2001 | 21.21415868233 | | 2000 | 20.937950075994 | | 1999 | 21.220667574185 | | 1998 | 20.708099265778 | | 1997 | 20.937769275375 | | 1996 | 21.479579252811 | | 1995 | 20.870164195092 | | 1994 | 21.846756252562 | | 1993 | 24.316065576892 | | 1992 | 23.636510086072 | | 1991 | 24.49111686464 | | 1990 | 23.286218111221 | | 1989 | 21.893389267404 | | 1988 | 22.609081693012 | | 1987 | 22.535240024829 | | 1986 | 21.258646679184 | | 1985 | 20.342250581474 | | 1984 | 20.157726304953 | | 1983 | 20.786086801346 | | 1982 | 21.685928049001 | | 1981 | 21.904118436085 | | 1980 | 23.074808562396 | | 1979 | 23.12002319418 | | 1978 | 23.189717581858 | | 1977 | 23.394929133125 | | 1976 | 22.304343847514 | | 1975 | 22.336712957068 | | 1974 | 21.216270749799 | | 1973 | 22.460625922139 |
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