Year	Years
1970	2.8
1971	2.9
1972	3
1973	3.1
1974	3.3
1975	3.4
1976	3.5
1977	3.7
1978	3.8
1979	3.9
1980	4.1
1981	4.2
1982	4.4
1983	4.5
1984	4.7
1985	4.8
1986	5
1987	5.1
1988	5.3
1989	5.4
1990	5.6
1991	5.7
1992	5.9
1993	6
1994	6.1
1995	6.3
1996	6.4
1997	6.6
1998	6.7
1999	6.8
2000	7
2001	7.1
2002	7.2
2003	7.3
2004	7.5
2005	7.6
2006	7.7
2007	7.8
2008	8
2009	8.1