| Year | Value | | 2017 | 4.1869918971707 | | 2016 | 3.9955384617932 | | 2015 | 3.8615585925418 | | 2014 | 3.7501252000748 | | 2013 | 3.6144968685622 | | 2012 | 3.525209263744 | | 2011 | 3.2487590182371 | | 2010 | 3.2058855555011 | | 2009 | 3.0846561285888 | | 2008 | 3.0090790936288 | | 2007 | 2.923922862611 | | 2006 | 2.8930957425056 | | 2005 | 2.8637999228112 | | 2004 | 2.9087215209998 | | 2003 | 2.8146166975278 | | 2002 | 2.9105966628199 | | 2001 | 2.7914440199403 | | 2000 | 2.8699076026932 | | 1999 | 3.5592957773789 | | 1998 | 3.6951605000076 | | 1997 | 3.5987190747953 | | 1996 | 3.6696580775959 | | 1995 | 3.8287423380204 | | 1994 | 3.9138711059399 | | 1993 | 3.9286612780669 | | 1992 | 3.9712948361849 | | 1991 | 4.0477203580298 | | 1990 | 3.8841094628432 | | 1989 | 4.0120395516011 | | 1988 | 4.1751695513426 | | 1987 | 4.0592964923902 | | 1986 | 4.0711233964865 | | 1985 | 4.0285232612873 | | 1981 | 2.68940586955 | | 1980 | 2.8120637672164 | | 1975 | 2.2410079577301 | | 1970 | 1.959207656293 |
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