| Year | Value | | 2022 | 8.2747749023265 | | 2021 | 3.4950575739945 | | 2020 | 1.6466637928638 | | 2019 | 1.8492298451109 | | 2018 | 2.611223782638 | | 2017 | 3.2053097163433 | | 2016 | 2.0452842085887 | | 2015 | 1.7864024395418 | | 2014 | 3.1059013062929 | | 2013 | 2.7448362189619 | | 2012 | 3.675918979745 | | 2011 | 5.2424857783188 | | 2010 | 3.6890122967077 | | 2009 | 2.5919467343057 | | 2008 | 9.8899978869186 | | 2007 | 5.5443848581516 | | 2006 | 4.4158998416956 | | 2005 | 4.5403691963453 | | 2004 | 4.7103706010588 | | 2003 | 4.5469001211872 | | 2002 | 5.7095158597663 | | 2001 | 6.0348191227038 | | 2000 | 4.8906255004257 | | 1999 | 5.0199701482527 | | 1998 | 6.8456698080724 | | 1997 | 7.1163646191633 | | 1996 | 9.7889204017997 | | 1995 | 13.425414364641 | | 1994 | 20.566194837635 | | 1993 | 13.075246755607 | | 1992 | 10.630199689304 | | 1991 | 14.402674880627 | | 1990 | 12.442217456296 | | 1989 | 9.2764955472518 | | 1988 | 8.7554968508614 | | 1987 | 8.256245831136 | | 1986 | 9.7221932767856 | | 1985 | 12.961686314347 | | 1984 | 9.5162519328545 | | 1983 | 10.541173715217 | | 1982 | 9.8521164045815 | | 1981 | 13.657328082689 | | 1978 | 9.5843791211129 | | 1977 | 11.151964086929 |
|