Year	Years
1970	2.4
1971	2.5
1972	2.5
1973	2.6
1974	2.7
1975	2.8
1976	2.8
1977	2.9
1978	3
1979	3.1
1980	3.2
1981	3.2
1982	3.3
1983	3.4
1984	3.5
1985	3.6
1986	3.7
1987	3.8
1988	3.8
1989	3.9
1990	4
1991	4.1
1992	4.2
1993	4.3
1994	4.4
1995	4.5
1996	4.6
1997	4.6
1998	4.7
1999	4.8
2000	4.9
2001	5
2002	5.1
2003	5.2
2004	5.3
2005	5.3
2006	5.4
2007	5.5
2008	5.6
2009	5.7