Year	Years
1970	3.7
1971	3.8
1972	4
1973	4.1
1974	4.2
1975	4.3
1976	4.5
1977	4.6
1978	4.7
1979	4.8
1980	5
1981	5.1
1982	5.2
1983	5.4
1984	5.5
1985	5.6
1986	5.8
1987	5.9
1988	6.1
1989	6.2
1990	6.3
1991	6.5
1992	6.6
1993	6.8
1994	6.9
1995	7.1
1996	7.2
1997	7.4
1998	7.5
1999	7.7
2000	7.8
2001	8
2002	8.1
2003	8.3
2004	8.4
2005	8.6
2006	8.7
2007	8.9
2008	9
2009	9.2