| Year | Value | | 2022 | 3.297 | | 2021 | 3.776 | | 2020 | 4.595 | | 2019 | 4.109 | | 2018 | 4.335 | | 2017 | 4.736 | | 2016 | 5.147 | | 2015 | 5.415 | | 2014 | 5.428 | | 2013 | 5.839 | | 2012 | 6.931 | | 2011 | 6.491 | | 2010 | 6.557 | | 2009 | 6.121 | | 2008 | 4.167 | | 2007 | 3.661 | | 2006 | 3.857 | | 2005 | 3.807 | | 2004 | 4.005 | | 2003 | 4.745 | | 2002 | 5.282 | | 2001 | 5.433 | | 2000 | 6.13 | | 1999 | 7.023 | | 1998 | 7.718 | | 1997 | 6.857 | | 1996 | 6.29 | | 1995 | 6.461 | | 1994 | 8.354 | | 1993 | 9.804 | | 1992 | 10.667 | | 1991 | 10.614 | | 1990 | 7.982 | | 1989 | 7.305 | | 1988 | 5.77 | | 1987 | 4.214 | | 1986 | 4.186 |
|