Year	Years
1970	2.2
1971	2.3
1972	2.4
1973	2.5
1974	2.6
1975	2.7
1976	2.8
1977	2.9
1978	3.1
1979	3.2
1980	3.3
1981	3.4
1982	3.6
1983	3.7
1984	3.8
1985	4
1986	4.1
1987	4.3
1988	4.4
1989	4.5
1990	4.7
1991	4.8
1992	5
1993	5.2
1994	5.3
1995	5.5
1996	5.6
1997	5.8
1998	5.9
1999	6.1
2000	6.3
2001	6.4
2002	6.6
2003	6.8
2004	6.9
2005	7.1
2006	7.2
2007	7.4
2008	7.6
2009	7.7