Year	Years
1970	2.5
1971	2.6
1972	2.7
1973	2.8
1974	2.9
1975	3
1976	3.1
1977	3.2
1978	3.3
1979	3.4
1980	3.5
1981	3.6
1982	3.7
1983	3.8
1984	3.9
1985	4
1986	4.1
1987	4.2
1988	4.3
1989	4.4
1990	4.6
1991	4.7
1992	4.8
1993	4.9
1994	5
1995	5.1
1996	5.2
1997	5.4
1998	5.5
1999	5.6
2000	5.7
2001	5.8
2002	6
2003	6.1
2004	6.2
2005	6.3
2006	6.4
2007	6.6
2008	6.7
2009	6.8