| Year | Value | | 2022 | 8.3064612444762 | | 2021 | 4.9284139694897 | | 2020 | 4.8716739856556 | | 2019 | 3.3934196962797 | | 2018 | 4.3086697912231 | | 2017 | 3.8925162536339 | | 2016 | 3.5231731762847 | | 2015 | 3.997379966944 | | 2014 | 5.1659023792517 | | 2013 | 5.3537995995712 | | 2012 | 5.9817213231556 | | 2011 | 6.5730826233045 | | 2010 | 4.8455187585212 | | 2009 | 5.0539305257559 | | 2008 | 11.581075174825 | | 2007 | 7.0255143697477 | | 2006 | 6.1568760393856 | | 2005 | 5.9550425007455 | | 2004 | 4.7358014394906 | | 2003 | 5.1289949942242 | | 2002 | 3.4328035378999 | | 2001 | 4.5673344028538 | | 2000 | 4.1654044229028 | | 1999 | 4.7785517406298 | | 1998 | 9.2349343226712 | | 1997 | 7.1642521146271 | | 1996 | 9.2753906045446 | | 1995 | 10.209046463415 | | 1994 | 13.256032591664 | | 1993 | 9.3703441597146 | | 1992 | 10.098967153517 | | 1991 | 13.224265772651 | | 1990 | 10.653505786249 | | 1989 | 9.3043612584317 | | 1988 | 8.9104702001338 | | 1987 | 8.2258064516129 | | 1986 | 8.7335591870667 | | 1985 | 7.7286389008592 | | 1984 | 10.983862571577 | | 1983 | 11.567524383407 | | 1982 | 10.527807297298 | | 1981 | 12.413261372398 | | 1980 | 14.271342116313 | | 1979 | 10.731921430873 | | 1978 | 7.346091250833 | | 1977 | 11.604138103012 |
|