Year	Years
1970	2.9
1971	3
1972	3.2
1973	3.3
1974	3.4
1975	3.5
1976	3.7
1977	3.8
1978	4
1979	4.1
1980	4.2
1981	4.4
1982	4.5
1983	4.7
1984	4.8
1985	5
1986	5.2
1987	5.3
1988	5.5
1989	5.6
1990	5.8
1991	6
1992	6.1
1993	6.3
1994	6.5
1995	6.6
1996	6.8
1997	6.9
1998	7.1
1999	7.3
2000	7.4
2001	7.6
2002	7.8
2003	7.9
2004	8.1
2005	8.2
2006	8.4
2007	8.6
2008	8.7
2009	8.9