Year	Years
1970	2.9
1971	3
1972	3.1
1973	3.2
1974	3.4
1975	3.5
1976	3.6
1977	3.7
1978	3.8
1979	4
1980	4.1
1981	4.2
1982	4.4
1983	4.5
1984	4.6
1985	4.8
1986	4.9
1987	5.1
1988	5.2
1989	5.4
1990	5.5
1991	5.7
1992	5.8
1993	6
1994	6.1
1995	6.3
1996	6.4
1997	6.6
1998	6.7
1999	6.9
2000	7
2001	7.2
2002	7.3
2003	7.4
2004	7.6
2005	7.7
2006	7.9
2007	8
2008	8.2
2009	8.3