Year	Years
1970	2.5
1971	2.7
1972	2.8
1973	2.9
1974	3.1
1975	3.2
1976	3.3
1977	3.5
1978	3.6
1979	3.8
1980	3.9
1981	4.1
1982	4.2
1983	4.4
1984	4.5
1985	4.7
1986	4.8
1987	5
1988	5.1
1989	5.2
1990	5.4
1991	5.5
1992	5.7
1993	5.8
1994	6
1995	6.2
1996	6.3
1997	6.5
1998	6.6
1999	6.8
2000	7
2001	7.1
2002	7.3
2003	7.5
2004	7.6
2005	7.8
2006	8
2007	8.2
2008	8.3
2009	8.5