| Year | Value | | 2020 | 3.836 | | 2019 | 3.751 | | 2018 | 3.661 | | 2017 | 3.588 | | 2016 | 3.517 | | 2015 | 3.471 | | 2014 | 3.407 | | 2013 | 3.301 | | 2012 | 3.248 | | 2011 | 3.131 | | 2010 | 2.963 | | 2009 | 2.922 | | 2008 | 2.868 | | 2007 | 2.792 | | 2006 | 2.799 | | 2005 | 2.717 | | 2004 | 2.662 | | 2003 | 2.632 | | 2002 | 2.59 | | 2001 | 2.525 | | 2000 | 2.447 | | 1999 | 2.38 | | 1991 | 2.597 | | 1990 | 2.507 | | 1979 | 1.8 | | 1978 | 1.8 | | 1977 | 1.7 | | 1976 | 1.7 | | 1975 | 1.6 | | 1974 | 1.5 | | 1973 | 1.4 | | 1972 | 1.4 | | 1971 | 1.3 | | 1970 | 1.2 | | 1969 | 1.2 | | 1968 | 1.2 | | 1967 | 1.2 | | 1966 | 1.2 | | 1965 | 1.2 | | 1964 | 1.1 | | 1963 | 1.1 | | 1962 | 1.1 | | 1961 | 1.1 | | 1960 | 1.1 |
|