Year	Years
1970	3.9
1971	4
1972	4.1
1973	4.3
1974	4.4
1975	4.5
1976	4.6
1977	4.7
1978	4.8
1979	4.9
1980	5
1981	5.1
1982	5.3
1983	5.4
1984	5.5
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	6.6
1995	6.7
1996	6.8
1997	6.9
1998	7
1999	7.1
2000	7.2
2001	7.3
2002	7.4
2003	7.5
2004	7.6
2005	7.7
2006	7.8
2007	7.9
2008	8
2009	8.1