Year	Years
1970	3.8
1971	3.9
1972	4
1973	4.1
1974	4.2
1975	4.4
1976	4.5
1977	4.6
1978	4.7
1979	4.9
1980	5
1981	5.1
1982	5.2
1983	5.4
1984	5.5
1985	5.6
1986	5.7
1987	5.9
1988	6
1989	6.1
1990	6.3
1991	6.4
1992	6.5
1993	6.6
1994	6.8
1995	6.9
1996	7
1997	7.1
1998	7.3
1999	7.4
2000	7.5
2001	7.6
2002	7.8
2003	7.9
2004	8
2005	8.1
2006	8.2
2007	8.3
2008	8.5
2009	8.6