Year	Years
1970	2.5
1971	2.6
1972	2.7
1973	2.8
1974	2.9
1975	3
1976	3.2
1977	3.3
1978	3.4
1979	3.5
1980	3.7
1981	3.8
1982	3.9
1983	4.1
1984	4.2
1985	4.3
1986	4.5
1987	4.6
1988	4.7
1989	4.9
1990	5
1991	5.1
1992	5.3
1993	5.4
1994	5.5
1995	5.7
1996	5.8
1997	6
1998	6.1
1999	6.2
2000	6.4
2001	6.5
2002	6.7
2003	6.8
2004	6.9
2005	7.1
2006	7.2
2007	7.4
2008	7.5
2009	7.6