| Year | Value | | 2022 | 1.8997965320275 | | 2021 | 2.1013086823995 | | 2020 | 2.035838355764 | | 2019 | 1.885387466677 | | 2018 | 1.97127204652 | | 2017 | 1.8136166913062 | | 2016 | 1.7245651146227 | | 2015 | 1.8205881364563 | | 2014 | 1.8074294556368 | | 2013 | 1.8165513018447 | | 2012 | 1.803669825249 | | 2011 | 1.7197489324165 | | 2010 | 1.8758078577762 | | 2009 | 1.8226133795889 | | 2008 | 1.8695246883209 | | 2007 | 1.7771670882851 | | 2006 | 2.0061780780067 | | 2005 | 2.0003858660104 | | 2004 | 2.0407280132366 | | 2003 | 2.237545321844 | | 2002 | 2.53014006009 | | 2001 | 2.5025776621772 | | 2000 | 2.4359772878704 | | 1999 | 2.3944609705068 | | 1998 | 2.2581336824168 | | 1997 | 2.3997207469136 | | 1996 | 2.8059383904223 | | 1995 | 2.7146366745293 | | 1994 | 2.5777055697501 | | 1993 | 2.7923984520878 | | 1992 | 2.8495148123428 | | 1991 | 2.3947121927081 | | 1990 | 3.5274016644927 | | 1989 | 3.5291267414333 | | 1988 | 3.1566469093988 | | 1987 | 3.0394222088474 | | 1986 | 4.1212801796743 | | 1985 | 4.3042206435437 | | 1984 | 4.5756457564576 | | 1983 | 5.381984036488 | | 1982 | 6.4493068937651 | | 1981 | 6.2554612790213 | | 1980 | 4.6961086286929 | | 1979 | 3.8004338394794 | | 1978 | 3.6210798577433 | | 1977 | 3.7459201606829 | | 1976 | 3.4815635385346 | | 1975 | 4.2860641685036 | | 1974 | 4.531456225253 | | 1973 | 3.7641546270988 | | 1972 | 2.1980676328502 |
|