| Year | Value | | 2018 | 11.720602412022 | | 2017 | 11.100497895543 | | 2016 | 10.890459893014 | | 2015 | 10.460842800621 | | 2014 | 9.8864596603428 | | 2013 | 10.804020162867 | | 2012 | 10.658054041178 | | 2011 | 10.087558987885 | | 2010 | 10.225454436553 | | 2009 | 9.7319353122857 | | 2008 | 10.741386208675 | | 2007 | 11.681698863997 | | 2006 | 10.858923790361 | | 2005 | 9.9577696898702 | | 2004 | 9.4913777796196 | | 2003 | 9.0516901433539 | | 2002 | 8.6941318826845 | | 2001 | 8.1876964518347 | | 2000 | 8.8658641886264 | | 1999 | 9.3107998475988 | | 1998 | 8.8139386922035 | | 1997 | 9.6184925118874 | | 1996 | 9.5069779101626 | | 1995 | 9.5335094504417 | | 1994 | 9.2643630552992 | | 1993 | 8.8401528037125 | | 1992 | 10.064157623365 | | 1991 | 10.425313656644 | | 1990 | 10.2848743778 | | 1989 | 10.8174857128 | | 1988 | 10.659078205491 | | 1987 | 10.411415391457 | | 1986 | 10.750048276106 | | 1985 | 10.313611191573 | | 1984 | 9.7285924175974 | | 1983 | 9.6476146463498 | | 1982 | 9.6351268402498 | | 1981 | 9.6724128392482 | | 1980 | 9.4828009527044 | | 1979 | 9.9670075211042 | | 1978 | 9.5421715215673 | | 1977 | 8.8568407294559 | | 1976 | 9.3801346231057 | | 1975 | 9.2305228798812 | | 1974 | 8.5588104693405 |
|