| Year | Value | | 2021 | 13.830971856168 | | 2020 | 15.162210395728 | | 2019 | 15.074636288417 | | 2018 | 15.55888057822 | | 2017 | 14.373625136536 | | 2016 | 16.723680002637 | | 2015 | 15.436615423803 | | 2014 | 15.449217850313 | | 2013 | 16.492915183116 | | 2012 | 16.464784878989 | | 2011 | 14.297907607041 | | 2010 | 12.821726667296 | | 2009 | 15.208653759249 | | 2008 | 12.87182722768 | | 2007 | 16.770024933887 | | 2006 | 18.054862235575 | | 2005 | 13.069162882215 | | 2004 | 13.878500105646 | | 2003 | 14.733979125003 | | 2002 | 11.67861110986 | | 2001 | 12.039759675937 | | 2000 | 9.971180916602 | | 1999 | 10.374795559395 | | 1998 | 14.318016600432 | | 1997 | 14.265818983267 | | 1996 | 9.8490506617198 | | 1995 | 10.498762069129 | | 1994 | 13.671332676839 | | 1993 | 8.2306978230772 | | 1992 | 11.269296460367 | | 1991 | 9.816903037773 | | 1990 | 12.229360090093 | | 1989 | 15.869192686543 | | 1988 | 7.6761595704421 | | 1987 | 10.73335135284 | | 1986 | 2.3739772787208 | | 1985 | -3.0180636686235 | | 1984 | -11.606573362201 | | 1983 | -8.937247039837 | | 1982 | -8.822445646646 | | 1981 | -9.3679177458574 | | 1980 | -6.9865676325104 | | 1979 | 8.6282067769504 | | 1978 | 5.7914200894476 | | 1977 | 17.533619421674 | | 1976 | 8.9184724771522 | | 1975 | 7.2763861505773 |
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