Year	%
1968	4.2
1969	4
1970	4.1
1971	3.9
1972	3.7
1973	4
1974	3.8
1975	3.9
1976	4
1977	4
1978	3.9
1979	4
1980	3.8
1981	3.6
1982	3.6
1983	3.6
1984	3.6
1985	3.5
1986	3.3
1987	3.2
1988	3.1
1989	3
1990	2.8
1991	2.8
1992	2.8
1993	2.8
1994	2.8
1995	2.8
1996	2.8
1997	2.8
1998	2.7
1999	2.7
2000	2.6
2001	2.7
2002	2.8
2003	2.9
2004	2.9
2005	2.8
2006	2.8
2007	2.8
2008	2.8
2009	2.8
2010	3
2011	3.1
2012	3
2013	3.1
2014	3
2015	3
2016	2.9
2017	2.9
2018	2.8
2019	2.8
2020	3
2021	3