| Year | Value | | 2017 | 1.3768023447529 | | 2016 | 1.4747310901966 | | 2015 | 1.4839953082925 | | 2014 | 1.5971081356415 | | 2010 | 1.5527595524103 | | 2009 | 1.5441629876904 | | 2008 | 1.6474619609135 | | 2007 | 1.6255286490546 | | 2006 | 1.6574446395 | | 2005 | 1.6285384923399 | | 2004 | 1.6124005889144 | | 2003 | 1.5790653491426 | | 2002 | 1.5834760552782 | | 2001 | 1.5035815434923 | | 2000 | 1.6060451416537 | | 1990 | 1.7846693262822 | | 1981 | 1.6885145495441 | | 1975 | 1.5716972228864 | | 1970 | 1.9006536019323 | | 1960 | 1.9222308122534 |
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