Year	Years
1970	2.1
1971	2.2
1972	2.3
1973	2.4
1974	2.5
1975	2.6
1976	2.7
1977	2.8
1978	3
1979	3.1
1980	3.2
1981	3.3
1982	3.5
1983	3.6
1984	3.8
1985	3.9
1986	4
1987	4.2
1988	4.3
1989	4.5
1990	4.6
1991	4.8
1992	4.9
1993	5.1
1994	5.2
1995	5.4
1996	5.5
1997	5.7
1998	5.8
1999	6
2000	6.1
2001	6.3
2002	6.4
2003	6.6
2004	6.7
2005	6.8
2006	7
2007	7.1
2008	7.3
2009	7.4