| Year | Value | | 2022 | 7.7136772690142 | | 2021 | 3.8780544241113 | | 2020 | 0.99333702283873 | | 2019 | 2.2521219135803 | | 2018 | 2.3241448536789 | | 2017 | 2.1920101096298 | | 2016 | 1.6503899443164 | | 2015 | 1.8614830286337 | | 2014 | 3.3834127274947 | | 2013 | 2.5286478853837 | | 2012 | 3.7818075467969 | | 2011 | 4.9777356370697 | | 2010 | 3.4912886748753 | | 2009 | 2.5004999505014 | | 2008 | 8.1081081081078 | | 2007 | 4.5780857237715 | | 2006 | 4.2519532197796 | | 2005 | 4.4405756568791 | | 2004 | 4.3803228653461 | | 2003 | 3.7337356616764 | | 2002 | 3.9501166372159 | | 2001 | 4.0550806134465 | | 2000 | 4.608229988726 | | 1999 | 3.4543979358751 | | 1998 | 5.1102495451756 | | 1997 | 6.9495660853412 | | 1996 | 7.3591178178878 | | 1995 | 10.193206763074 | | 1994 | 9.7002503351631 | | 1993 | 10.263623749655 | | 1992 | 10.630199689304 | | 1991 | 21.615741122445 | | 1990 | 21.507988173268 | | 1989 | 11.409632006576 | | 1988 | 8.0126289057241 | | 1987 | 11.537536133789 | | 1986 | 9.7643692434206 | | 1985 | 12.961686314347 | | 1984 | 11.507169505731 | | 1983 | 10.243663352654 | | 1982 | 7.8127414121649 | | 1981 | 14.047968671887 | | 1980 | 17.919924140627 | | 1979 | 14.722170368999 | | 1978 | 10.306129222296 | | 1977 | 10.688957420775 | | 1976 | 9.7936532753465 | | 1975 | 16.773570324582 | | 1974 | 23.244048532597 | | 1973 | 13.618317345838 | | 1972 | 6.6711776652837 | | 1971 | 4.7841143815462 | | 1970 | 4.6522974930315 | | 1969 | 2.6293408365833 | | 1968 | 4.0938914310684 | | 1967 | 3.0169484750215 |
|