Year	Years
1970	2.4
1971	2.5
1972	2.6
1973	2.7
1974	2.8
1975	2.9
1976	3
1977	3.1
1978	3.2
1979	3.4
1980	3.5
1981	3.6
1982	3.8
1983	3.9
1984	4.1
1985	4.2
1986	4.4
1987	4.6
1988	4.7
1989	4.9
1990	5
1991	5.2
1992	5.4
1993	5.5
1994	5.7
1995	5.9
1996	6
1997	6.2
1998	6.4
1999	6.5
2000	6.7
2001	6.9
2002	7
2003	7.2
2004	7.3
2005	7.5
2006	7.7
2007	7.8
2008	8
2009	8.1