| Year | Value | | 2017 | 4.5154222382576 | | 2016 | 4.2629899041108 | | 2015 | 4.1856960683237 | | 2014 | 3.8527051765733 | | 2013 | 3.6837318629369 | | 2012 | 3.4981609775022 | | 2011 | 3.2136058875076 | | 2010 | 3.142580371641 | | 2009 | 2.9538257542925 | | 2008 | 3.1642920313635 | | 2007 | 3.0415547411982 | | 2006 | 2.9023864195509 | | 2005 | 2.9433543998448 | | 2004 | 2.9693572348802 | | 2003 | 2.9461503742166 | | 2002 | 2.5649889208599 | | 2001 | 2.7536652995375 | | 2000 | 2.8211278174961 | | 1999 | 3.7002016560925 | | 1998 | 3.3892572721742 | | 1997 | 3.7082878158064 | | 1996 | 3.8037154348979 | | 1995 | 3.8017554555608 | | 1994 | 3.5270344527211 | | 1993 | 3.4367797299182 | | 1992 | 2.4287046045804 | | 1991 | 2.4209031049061 | | 1990 | 2.3408509869245 | | 1989 | 2.4857748553602 | | 1986 | 2.3966326593463 | | 1985 | 3.2848934706694 | | 1984 | 2.4546544286881 | | 1983 | 2.4114674436734 | | 1982 | 2.3824810928945 | | 1981 | 2.4382424774377 | | 1980 | 3.3261601479412 | | 1976 | 2.1278020849699 | | 1975 | 1.9157766403878 | | 1970 | 2.5504340996637 |
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