Year	Years
1970	3.1
1971	3.2
1972	3.3
1973	3.4
1974	3.5
1975	3.6
1976	3.8
1977	3.9
1978	4
1979	4.1
1980	4.2
1981	4.3
1982	4.4
1983	4.5
1984	4.7
1985	4.8
1986	4.9
1987	5
1988	5.1
1989	5.2
1990	5.3
1991	5.5
1992	5.6
1993	5.7
1994	5.8
1995	5.9
1996	6
1997	6.2
1998	6.3
1999	6.4
2000	6.5
2001	6.6
2002	6.7
2003	6.9
2004	7
2005	7.1
2006	7.2
2007	7.3
2008	7.5
2009	7.6