Year	Years
1970	3.6
1971	3.7
1972	3.8
1973	3.9
1974	4
1975	4.2
1976	4.3
1977	4.4
1978	4.5
1979	4.6
1980	4.8
1981	4.9
1982	5.1
1983	5.2
1984	5.4
1985	5.6
1986	5.8
1987	6
1988	6.2
1989	6.4
1990	6.5
1991	6.7
1992	6.9
1993	7.1
1994	7.2
1995	7.4
1996	7.6
1997	7.7
1998	7.9
1999	8
2000	8.2
2001	8.3
2002	8.5
2003	8.6
2004	8.8
2005	8.9
2006	9
2007	9.1
2008	9.3
2009	9.4