Year	Years
1970	3.1
1971	3.2
1972	3.3
1973	3.4
1974	3.5
1975	3.6
1976	3.7
1977	3.9
1978	4
1979	4.1
1980	4.2
1981	4.3
1982	4.5
1983	4.6
1984	4.7
1985	4.8
1986	5
1987	5.1
1988	5.2
1989	5.4
1990	5.5
1991	5.6
1992	5.8
1993	5.9
1994	6
1995	6.2
1996	6.3
1997	6.4
1998	6.6
1999	6.7
2000	6.8
2001	7
2002	7.1
2003	7.3
2004	7.4
2005	7.5
2006	7.7
2007	7.8
2008	7.9
2009	8.1