Year	Years
1970	2.6
1971	2.7
1972	2.8
1973	2.9
1974	3
1975	3.1
1976	3.2
1977	3.4
1978	3.5
1979	3.6
1980	3.7
1981	3.9
1982	4
1983	4.1
1984	4.2
1985	4.3
1986	4.5
1987	4.6
1988	4.7
1989	4.8
1990	5
1991	5.1
1992	5.2
1993	5.4
1994	5.5
1995	5.6
1996	5.8
1997	5.9
1998	6
1999	6.2
2000	6.3
2001	6.4
2002	6.6
2003	6.7
2004	6.8
2005	7
2006	7.1
2007	7.2
2008	7.4
2009	7.5