| Year | Value | | 2021 | 7.379559049139 | | 2020 | 7.3797460025005 | | 2019 | 7.3865692758201 | | 2018 | 7.3802316399052 | | 2017 | 7.3769413003422 | | 2016 | 7.3903458260669 | | 2015 | 7.3781711690961 | | 2014 | 7.3408141103689 | | 2013 | 7.3187666589884 | | 2012 | 7.4010332686169 | | 2011 | 7.4056401987561 | | 2010 | 7.4195459032577 | | 2009 | 7.4310628496216 | | 2008 | 7.4336952945048 | | 2007 | 7.4211582097405 | | 2006 | 7.4331656308573 | | 2005 | 7.3426573426573 | | 2004 | 7.2907843330379 | | 2003 | 7.2681309301028 | | 2002 | 7.2395679437933 | | 2001 | 7.2008273416724 | | 2000 | 7.1837552119242 | | 1999 | 7.1476410913031 | | 1998 | 7.115138382744 | | 1997 | 6.9785613447585 | | 1996 | 6.9713385206343 | | 1995 | 7.0373288683148 | | 1994 | 7.5501493811353 | | 1993 | 7.4710266259562 | | 1992 | 7.4999179224531 | | 1991 | 7.5501493811353 | | 1990 | 7.449029843396 | | 1989 | 7.4221084080239 | | 1988 | 7.3751600512164 | | 1987 | 7.4828457927049 | | 1986 | 7.4184969959618 | | 1985 | 7.4684001444565 | | 1984 | 7.5173183623888 | | 1983 | 7.5885616730687 | | 1982 | 7.6322269280016 | | 1981 | 7.7241537804918 | | 1980 | 7.7776683410486 | | 1979 | 7.8778029482255 | | 1978 | 7.9126038280968 | | 1977 | 7.9405101940313 | | 1976 | 8.0826685052037 | | 1975 | 8.0495091762697 | | 1974 | 8.028497324272 |
|