Year	Years
1970	3.8
1971	3.9
1972	4
1973	4.2
1974	4.3
1975	4.4
1976	4.5
1977	4.6
1978	4.7
1979	4.8
1980	4.9
1981	5
1982	5.1
1983	5.2
1984	5.3
1985	5.4
1986	5.5
1987	5.6
1988	5.7
1989	5.8
1990	5.9
1991	6
1992	6.1
1993	6.2
1994	6.3
1995	6.4
1996	6.5
1997	6.6
1998	6.7
1999	6.8
2000	6.9
2001	7
2002	7.1
2003	7.1
2004	7.2
2005	7.3
2006	7.4
2007	7.5
2008	7.6
2009	7.7