Year	Years
1970	3.3
1971	3.4
1972	3.5
1973	3.7
1974	3.8
1975	3.9
1976	4.1
1977	4.2
1978	4.3
1979	4.4
1980	4.6
1981	4.7
1982	4.8
1983	5
1984	5.1
1985	5.2
1986	5.4
1987	5.5
1988	5.7
1989	5.8
1990	5.9
1991	6.1
1992	6.2
1993	6.3
1994	6.5
1995	6.6
1996	6.7
1997	6.9
1998	7
1999	7.1
2000	7.3
2001	7.4
2002	7.5
2003	7.6
2004	7.8
2005	7.9
2006	8
2007	8.2
2008	8.3
2009	8.4