Year	Years
1970	3.4
1971	3.5
1972	3.6
1973	3.7
1974	3.9
1975	4
1976	4.2
1977	4.3
1978	4.4
1979	4.6
1980	4.7
1981	4.9
1982	5
1983	5.1
1984	5.3
1985	5.4
1986	5.6
1987	5.7
1988	5.9
1989	6
1990	6.2
1991	6.3
1992	6.5
1993	6.6
1994	6.8
1995	6.9
1996	7.1
1997	7.3
1998	7.4
1999	7.6
2000	7.8
2001	8
2002	8.1
2003	8.3
2004	8.5
2005	8.7
2006	8.8
2007	9
2008	9.2
2009	9.4