Year	Years
1970	3.7
1971	3.8
1972	3.9
1973	4
1974	4.1
1975	4.2
1976	4.4
1977	4.5
1978	4.6
1979	4.7
1980	4.8
1981	4.9
1982	5.1
1983	5.2
1984	5.3
1985	5.4
1986	5.6
1987	5.7
1988	5.8
1989	6
1990	6.1
1991	6.2
1992	6.3
1993	6.4
1994	6.6
1995	6.7
1996	6.8
1997	6.9
1998	7.1
1999	7.2
2000	7.3
2001	7.4
2002	7.5
2003	7.6
2004	7.7
2005	7.9
2006	8
2007	8.1
2008	8.2
2009	8.3