Year	Years
1970	4.2
1971	4.4
1972	4.5
1973	4.6
1974	4.8
1975	4.9
1976	5.1
1977	5.2
1978	5.4
1979	5.5
1980	5.7
1981	5.9
1982	6
1983	6.2
1984	6.4
1985	6.6
1986	6.7
1987	6.9
1988	7.1
1989	7.3
1990	7.4
1991	7.6
1992	7.8
1993	8
1994	8.1
1995	8.3
1996	8.4
1997	8.6
1998	8.8
1999	8.9
2000	9.1
2001	9.2
2002	9.4
2003	9.5
2004	9.7
2005	9.8
2006	10
2007	10.1
2008	10.2
2009	10.4