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	4.8
1980	5
1981	5.1
1982	5.2
1983	5.3
1984	5.4
1985	5.6
1986	5.7
1987	5.8
1988	5.9
1989	6.1
1990	6.2
1991	6.3
1992	6.4
1993	6.5
1994	6.7
1995	6.8
1996	6.9
1997	7
1998	7.1
1999	7.3
2000	7.4
2001	7.5
2002	7.6
2003	7.8
2004	7.9
2005	8
2006	8.1
2007	8.2
2008	8.4
2009	8.5