Year	Years
1970	2.9
1971	3
1972	3.1
1973	3.2
1974	3.3
1975	3.4
1976	3.5
1977	3.6
1978	3.7
1979	3.8
1980	3.9
1981	4
1982	4.1
1983	4.2
1984	4.3
1985	4.4
1986	4.5
1987	4.6
1988	4.7
1989	4.8
1990	4.9
1991	5
1992	5.2
1993	5.3
1994	5.4
1995	5.5
1996	5.6
1997	5.7
1998	5.8
1999	5.9
2000	6
2001	6.2
2002	6.3
2003	6.4
2004	6.5
2005	6.6
2006	6.7
2007	6.8
2008	6.9
2009	7.1