| Year | Value | | 2022 | 9.4595525303968 | | 2021 | 4.2593641754287 | | 2020 | 3.8428643984187 | | 2019 | 2.8027030792811 | | 2018 | 4.0929781930063 | | 2017 | 5.1969818534017 | | 2016 | 5.4305196611917 | | 2015 | 3.5507596735675 | | 2014 | 4.4022530924343 | | 2013 | 4.8851638759547 | | 2012 | 6.5818310507512 | | 2011 | 5.3662109294808 | | 2010 | 4.0331413897432 | | 2009 | 5.0062736153405 | | 2008 | 10.378524478798 | | 2007 | 6.7866978903824 | | 2006 | 6.537843952996 | | 2005 | 6.4275527807009 | | 2004 | 4.1366321314475 | | 2003 | 5.6794177109911 | | 2002 | 4.7057912374603 | | 2001 | 5.1474680022262 | | 2000 | 4.500788562621 | | 1999 | 4.3564337451045 | | 1998 | 6.4357328756078 | | 1997 | 7.6471743331001 | | 1996 | 7.1916466043334 | | 1995 | 11.425939964662 | | 1994 | 27.447980712695 | | 1993 | 9.5248453701969 | | 1992 | 9.4865425061716 | | 1991 | 8.9343065693429 | | 1990 | 7.5448610969945 | | 1989 | 8.0461586348732 | | 1988 | 8.3516795682122 | | 1987 | 7.632790088781 | | 1986 | 6.8593442279489 | | 1985 | 9.4069075068727 | | 1984 | 11.116931285788 | | 1983 | 11.482653560129 | | 1982 | 12.099841451803 | | 1981 | 12.290327860596 | | 1980 | 13.601982828949 | | 1979 | 13.674625454436 | | 1978 | 11.459429442539 | | 1977 | 14.890066502322 | | 1976 | 11.544880651984 | | 1975 | 18.31178575399 |
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