Year	Years
1970	4.1
1971	4.2
1972	4.3
1973	4.4
1974	4.5
1975	4.7
1976	4.8
1977	4.9
1978	5
1979	5.1
1980	5.2
1981	5.3
1982	5.4
1983	5.5
1984	5.6
1985	5.8
1986	5.9
1987	6
1988	6.1
1989	6.2
1990	6.4
1991	6.5
1992	6.6
1993	6.7
1994	6.8
1995	6.9
1996	7
1997	7.2
1998	7.3
1999	7.4
2000	7.5
2001	7.6
2002	7.7
2003	7.8
2004	7.9
2005	8
2006	8.1
2007	8.2
2008	8.3
2009	8.4