Year	Years
1970	3.1
1971	3.2
1972	3.3
1973	3.5
1974	3.6
1975	3.7
1976	3.8
1977	3.9
1978	4
1979	4.1
1980	4.3
1981	4.4
1982	4.5
1983	4.6
1984	4.7
1985	4.8
1986	5
1987	5.1
1988	5.2
1989	5.3
1990	5.4
1991	5.5
1992	5.7
1993	5.8
1994	5.9
1995	6
1996	6.1
1997	6.3
1998	6.4
1999	6.5
2000	6.6
2001	6.7
2002	6.9
2003	7
2004	7.1
2005	7.2
2006	7.3
2007	7.4
2008	7.6
2009	7.7