| Year | Value | | 2022 | 6.2768931944754 | | 2021 | 2.4107525528876 | | 2020 | 0.63226733191172 | | 2019 | 1.5174494141115 | | 2018 | 2.2800532685749 | | 2017 | 1.8203999968206 | | 2016 | 1.129801632669 | | 2015 | 0.96110005880162 | | 2014 | 1.6416298257028 | | 2013 | 1.9234360410831 | | 2012 | 3.6354004808047 | | 2011 | 4.7958826618311 | | 2010 | 2.99936630549 | | 2009 | 2.5160912814511 | | 2008 | 8.758727695888 | | 2007 | 4.965976545533 | | 2006 | 4.0940579095392 | | 2005 | 3.475806937554 | | 2004 | 3.1581932429846 | | 2003 | 2.4451956817822 | | 2002 | 2.6282560973323 | | 2001 | 3.414789461116 | | 2000 | 3.0674396693059 | | 1999 | 2.9537083606913 | | 1998 | 3.2381944796978 | | 1997 | 3.6157116010385 | | 1996 | 3.2273681202471 | | 1995 | 5.3355333634439 | | 1994 | 3.4520583301071 | | 1993 | 4.2913755344665 | | 1992 | 5.305361696445 | | 1991 | 6.3633312175847 | | 1990 | 7.6657107329255 | | 1989 | 6.1897340511324 | | 1988 | 5.6576097230799 | | 1987 | 4.3279829713902 | | 1986 | 5.09996143777 | | 1985 | 4.8197316487797 | | 1984 | 5.4038088595773 | | 1983 | 5.5894980184394 | | 1982 | 7.2764086177986 | | 1981 | 12.413261372398 | | 1980 | 14.492874406452 | | 1978 | 8.8553459121409 | | 1977 | 11.191001637036 |
|