| Year | Value | | 2022 | 11.517932578424 | | 2021 | 11.163902089101 | | 2020 | 11.020714321671 | | 2019 | 10.877526554241 | | 2018 | 10.734338786811 | | 2017 | 10.591151019381 | | 2016 | 10.447963251951 | | 2015 | 10.30477548452 | | 2014 | 10.161587968789 | | 2013 | 9.9880835640574 | | 2012 | 9.8145794110244 | | 2011 | 9.6410750062925 | | 2010 | 9.4675708532595 | | 2009 | 9.2940667002265 | | 2008 | 9.1205622954946 | | 2007 | 8.9470581424616 | | 2006 | 8.7735539894286 | | 2005 | 8.6000495846967 | | 2004 | 8.4265454316637 | | 2003 | 8.2530410269318 | | 2002 | 8.0795368738988 | | 2001 | 7.8152529574629 | | 2000 | 7.6013088346338 | | 1999 | 7.4251195570098 | | 1998 | 7.1734205889756 | | 1997 | 6.54417316889 | | 1996 | 6.54417316889 | | 1995 | 6.54417316889 | | 1994 | 6.1666247168387 | | 1993 | 5.7890762647873 | | 1992 | 5.7890762647873 | | 1991 | 5.411527812736 | | 1990 | 5.3108482255223 | | 1989 | 5.2101686383086 | | 1988 | 5.1094890510949 | | 1987 | 5.0088094638812 | | 1986 | 4.9081298766675 | | 1985 | 4.7822803926504 | | 1984 | 4.6816008054367 | | 1983 | 4.580921218223 | | 1982 | 4.4802416310093 | | 1981 | 4.382079033476 | | 1980 | 4.0775232821545 | | 1979 | 3.7754845205135 | | 1978 | 3.4986156556758 | | 1977 | 3.1965768940347 | | 1976 | 2.9197080291971 | | 1975 | 2.617669267556 |
|