| Year | Value | | 2022 | 17.972508594771 | | 2021 | 19.050308581315 | | 2020 | 18.532917284614 | | 2019 | 16.671689941195 | | 2018 | 14.666289634463 | | 2017 | 14.784342955503 | | 2016 | 14.823944564275 | | 2015 | 13.575192207547 | | 2014 | 11.771015460302 | | 2013 | 11.599704619808 | | 2012 | 11.558543262384 | | 2011 | 11.128842900004 | | 2010 | 11.647159011479 | | 2009 | 12.79914146631 | | 2008 | 11.604735700785 | | 2007 | 14.455074545749 | | 2006 | 14.358038997166 | | 2005 | 13.802106763489 | | 2004 | 15.667990279835 | | 2003 | 14.337354117769 | | 2002 | 13.720080445351 | | 2001 | 9.9752218496035 | | 2000 | 12.47922174157 | | 1999 | 11.789601814572 | | 1998 | 12.004280060912 | | 1997 | 10.113379489669 | | 1996 | 9.7807471406667 | | 1995 | 11.149828140931 | | 1994 | 10.804232741291 | | 1993 | 11.020779797989 | | 1992 | 11.484721036292 | | 1991 | 12.348292284467 | | 1990 | 10.197287556451 | | 1989 | 9.1854940272202 | | 1988 | 8.4787464145655 | | 1987 | 7.6630299285874 | | 1986 | 9.8663568009448 | | 1985 | 8.9145681885756 | | 1984 | 7.5448763722822 | | 1983 | 8.6066316637953 | | 1982 | 9.6604282319753 | | 1981 | 10.580309825839 | | 1980 | 13.966772437077 | | 1979 | 16.115671888399 | | 1978 | 17.448030921226 | | 1977 | 19.505520682218 |
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