Year	Years
1970	2.5
1971	2.6
1972	2.7
1973	2.8
1974	2.9
1975	3
1976	3.1
1977	3.3
1978	3.4
1979	3.5
1980	3.6
1981	3.7
1982	3.8
1983	4
1984	4.1
1985	4.2
1986	4.3
1987	4.5
1988	4.6
1989	4.7
1990	4.9
1991	5
1992	5.1
1993	5.3
1994	5.4
1995	5.6
1996	5.7
1997	5.8
1998	6
1999	6.1
2000	6.3
2001	6.4
2002	6.6
2003	6.7
2004	6.8
2005	7
2006	7.1
2007	7.3
2008	7.4
2009	7.6