| Year | Value | | 2019 | 3.737 | | 2018 | 3.996 | | 2017 | 4.33 | | 2016 | 4.81 | | 2015 | 5.301 | | 2014 | 6.11 | | 2013 | 7.525 | | 2012 | 7.885 | | 2011 | 8.038 | | 2010 | 7.787 | | 2009 | 7.537 | | 2008 | 5.615 | | 2007 | 5.262 | | 2006 | 5.35 | | 2005 | 4.751 | | 2004 | 4.594 | | 2003 | 4.807 | | 2002 | 5.037 | | 2001 | 4.696 | | 2000 | 5.558 | | 1999 | 6.042 | | 1998 | 6.198 | | 1997 | 7.072 | | 1996 | 8.189 | | 1995 | 8.694 | | 1994 | 9.648 | | 1993 | 10.346 | | 1992 | 9.772 | | 1991 | 8.55 | | 1990 | 6.969 | | 1989 | 7.405 | | 1988 | 9.004 | | 1987 | 11.017 | | 1986 | 11.508 | | 1985 | 11.48 | | 1984 | 10.901 | | 1983 | 11.088 | | 1982 | 10.9 | | 1981 | 10.4 | | 1980 | 6.8 | | 1979 | 5.3 | | 1978 | 5.7 | | 1977 | 5.8 | | 1976 | 5.5 | | 1975 | 4 | | 1974 | 2.6 | | 1973 | 2.6 | | 1972 | 3.7 | | 1971 | 3.3 |
|