Year	Years
1970	2.8
1971	2.9
1972	3
1973	3.1
1974	3.2
1975	3.3
1976	3.4
1977	3.5
1978	3.7
1979	3.8
1980	3.9
1981	4
1982	4.1
1983	4.3
1984	4.4
1985	4.5
1986	4.6
1987	4.7
1988	4.8
1989	5
1990	5.1
1991	5.2
1992	5.3
1993	5.4
1994	5.5
1995	5.7
1996	5.8
1997	5.9
1998	6
1999	6.1
2000	6.3
2001	6.4
2002	6.5
2003	6.6
2004	6.7
2005	6.8
2006	7
2007	7.1
2008	7.2
2009	7.3