Year	Years
1970	3.4
1971	3.5
1972	3.7
1973	3.8
1974	3.9
1975	4.1
1976	4.2
1977	4.3
1978	4.5
1979	4.6
1980	4.7
1981	4.8
1982	5
1983	5.1
1984	5.2
1985	5.4
1986	5.5
1987	5.6
1988	5.7
1989	5.9
1990	6
1991	6.1
1992	6.2
1993	6.4
1994	6.5
1995	6.6
1996	6.8
1997	6.9
1998	7
1999	7.2
2000	7.3
2001	7.4
2002	7.6
2003	7.7
2004	7.8
2005	8
2006	8.1
2007	8.2
2008	8.4
2009	8.5