| Year | Value | | 2022 | 9.4595525303968 | | 2021 | 4.2593641754287 | | 2020 | 3.4839356749321 | | 2019 | 2.835145556875 | | 2018 | 4.2746219861055 | | 2017 | 5.2097170591421 | | 2016 | 5.4941046376007 | | 2015 | 3.4723875288365 | | 2014 | 4.4038027171436 | | 2013 | 4.9052087569456 | | 2012 | 6.5758997075766 | | 2011 | 5.6863195957595 | | 2010 | 4.0897298947046 | | 2009 | 4.8486696464071 | | 2008 | 10.296975822125 | | 2007 | 7.0255143697477 | | 2006 | 6.6165543053648 | | 2005 | 6.605240855492 | | 2004 | 4.1781600011979 | | 2003 | 6.0844248988622 | | 2002 | 5.032822757112 | | 2001 | 5.077450455225 | | 2000 | 4.1992882562281 | | 1999 | 4.2495525589725 | | 1998 | 6.6613986566575 | | 1997 | 8.1690214410696 | | 1996 | 7.2728799957015 | | 1995 | 12.288591260143 | | 1994 | 28.814389430673 | | 1993 | 9.6793465806793 | | 1992 | 9.5234771910899 | | 1991 | 8.9656226521431 | | 1990 | 7.7253218884121 | | 1989 | 8.2750160875159 | | 1988 | 8.7554968508614 | | 1987 | 8.1523425380087 | | 1986 | 7.4333333333332 | | 1985 | 10.305440669829 | | 1984 | 11.25 | | 1983 | 11.567524383407 | | 1982 | 12.139917695473 | | 1981 | 12.413261372398 |
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