Year	Years
1970	3.5
1971	3.7
1972	3.8
1973	4
1974	4.2
1975	4.3
1976	4.5
1977	4.7
1978	4.9
1979	5.1
1980	5.2
1981	5.4
1982	5.6
1983	5.8
1984	6
1985	6.2
1986	6.4
1987	6.6
1988	6.8
1989	7
1990	7.2
1991	7.4
1992	7.6
1993	7.8
1994	7.9
1995	8.1
1996	8.3
1997	8.5
1998	8.7
1999	8.9
2000	9
2001	9.2
2002	9.4
2003	9.6
2004	9.7
2005	9.9
2006	10.1
2007	10.2
2008	10.4
2009	10.6