Year	Years
1970	3.3
1971	3.4
1972	3.6
1973	3.7
1974	3.9
1975	4
1976	4.2
1977	4.4
1978	4.6
1979	4.7
1980	4.9
1981	5.1
1982	5.3
1983	5.5
1984	5.7
1985	5.9
1986	6.2
1987	6.4
1988	6.6
1989	6.8
1990	7
1991	7.2
1992	7.5
1993	7.7
1994	7.9
1995	8.1
1996	8.3
1997	8.5
1998	8.7
1999	8.9
2000	9.1
2001	9.3
2002	9.5
2003	9.7
2004	9.9
2005	10.1
2006	10.3
2007	10.5
2008	10.7
2009	10.8