| Year | Value | | 2021 | 14.501679731243 | | 2020 | 14.501679731243 | | 2019 | 14.352370287421 | | 2018 | 14.19372900336 | | 2017 | 14.035087719298 | | 2016 | 13.876446435237 | | 2015 | 13.717805151176 | | 2014 | 13.184957073535 | | 2013 | 12.868607689436 | | 2012 | 13.245614035088 | | 2011 | 13.622620380739 | | 2010 | 13.99962672639 | | 2009 | 14.376633072042 | | 2008 | 14.753639417693 | | 2007 | 14.706980216499 | | 2006 | 14.053751399776 | | 2005 | 13.064576334453 | | 2004 | 15.574841358716 | | 2003 | 18.085106382979 | | 2002 | 17.315976110489 | | 2001 | 16.546752519597 | | 2000 | 15.777528928705 | | 1999 | 15.008305337813 | | 1998 | 14.23908174692 | | 1997 | 13.469858156028 | | 1996 | 12.700634565136 | | 1995 | 12.644643523703 | | 1994 | 12.635311683464 | | 1993 | 12.355356476297 | | 1992 | 12.131392310564 | | 1991 | 12.131392310564 | | 1990 | 12.131392310564 | | 1989 | 12.131392310564 | | 1988 | 12.131392310564 | | 1987 | 12.131392310564 | | 1986 | 12.131392310564 | | 1985 | 12.131392310564 | | 1984 | 12.131392310564 | | 1983 | 12.131392310564 | | 1982 | 12.131392310564 | | 1981 | 11.898096304591 | | 1980 | 11.851437103397 | | 1979 | 11.65546845838 | | 1978 | 11.664800298619 | | 1977 | 11.664800298619 | | 1976 | 11.198208286674 | | 1975 | 10.918253079507 | | 1974 | 10.918253079507 | | 1973 | 10.731616274729 | | 1972 | 10.451661067562 | | 1971 | 10.265024262785 | | 1970 | 10.265024262785 | | 1969 | 10.265024262785 | | 1968 | 10.358342665174 | | 1967 | 10.358342665174 | | 1966 | 10.265024262785 | | 1965 | 10.265024262785 | | 1964 | 10.265024262785 | | 1963 | 10.265024262785 | | 1962 | 10.265024262785 | | 1961 | 10.265024262785 |
|