Year	Years
1970	2.5
1971	2.6
1972	2.7
1973	2.8
1974	2.9
1975	3.1
1976	3.2
1977	3.3
1978	3.4
1979	3.6
1980	3.7
1981	3.8
1982	4
1983	4.1
1984	4.2
1985	4.4
1986	4.5
1987	4.7
1988	4.8
1989	4.9
1990	5.1
1991	5.2
1992	5.4
1993	5.5
1994	5.6
1995	5.8
1996	5.9
1997	6.1
1998	6.2
1999	6.4
2000	6.5
2001	6.7
2002	6.8
2003	7
2004	7.1
2005	7.2
2006	7.4
2007	7.5
2008	7.7
2009	7.8