Year	Years
1970	2.8
1971	2.9
1972	3
1973	3.1
1974	3.3
1975	3.4
1976	3.5
1977	3.6
1978	3.7
1979	3.8
1980	4
1981	4.1
1982	4.2
1983	4.3
1984	4.5
1985	4.6
1986	4.8
1987	4.9
1988	5.1
1989	5.2
1990	5.4
1991	5.6
1992	5.7
1993	5.9
1994	6
1995	6.2
1996	6.4
1997	6.5
1998	6.7
1999	6.8
2000	7
2001	7.2
2002	7.3
2003	7.5
2004	7.7
2005	7.8
2006	8
2007	8.1
2008	8.3
2009	8.5