Year	Years
1970	2.5
1971	2.6
1972	2.6
1973	2.7
1974	2.7
1975	2.8
1976	2.9
1977	2.9
1978	3
1979	3
1980	3
1981	3.1
1982	3.1
1983	3.2
1984	3.2
1985	3.2
1986	3.3
1987	3.3
1988	3.3
1989	3.4
1990	3.4
1991	3.4
1992	3.5
1993	3.5
1994	3.6
1995	3.6
1996	3.6
1997	3.7
1998	3.7
1999	3.7
2000	3.8
2001	3.8
2002	3.9
2003	3.9
2004	3.9
2005	4
2006	4
2007	4.1
2008	4.1
2009	4.1