| Year | Value | | 2020 | 3.324 | | 2019 | 3.311 | | 2018 | 3.276 | | 2017 | 3.275 | | 2016 | 3.255 | | 2015 | 3.24 | | 2014 | 3.225 | | 2013 | 3.209 | | 2012 | 3.188 | | 2011 | 3.172 | | 2010 | 3.397 | | 2009 | 3.394 | | 2008 | 3.443 | | 2007 | 3.449 | | 2006 | 3.469 | | 2005 | 3.476 | | 2004 | 3.473 | | 2003 | 3.454 | | 2002 | 3.435 | | 2001 | 3.408 | | 2000 | 3.375 | | 1999 | 3.355 | | 1998 | 3.355 | | 1997 | 3.343 | | 1996 | 3.326 | | 1995 | 3.307 | | 1994 | 3.286 | | 1993 | 3.251 | | 1992 | 3.223 | | 1991 | 3.171 | | 1990 | 3.118 | | 1989 | 3 | | 1988 | 2.9 | | 1987 | 2.8 | | 1986 | 2.8 | | 1985 | 2.7 | | 1984 | 2.5 | | 1983 | 2.1 | | 1982 | 2.1 | | 1981 | 2 | | 1980 | 1.9 | | 1979 | 1.8 | | 1978 | 1.7 | | 1977 | 1.6 | | 1976 | 1.5 | | 1975 | 1.5 | | 1974 | 1.4 | | 1973 | 1.4 | | 1972 | 1.3 | | 1971 | 1.3 | | 1970 | 1.2 |
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