Year	Years
1970	2.7
1971	2.8
1972	2.9
1973	3
1974	3.1
1975	3.3
1976	3.4
1977	3.5
1978	3.6
1979	3.8
1980	3.9
1981	4
1982	4.2
1983	4.3
1984	4.4
1985	4.6
1986	4.7
1987	4.9
1988	5
1989	5.1
1990	5.3
1991	5.4
1992	5.5
1993	5.6
1994	5.8
1995	5.9
1996	6
1997	6.2
1998	6.3
1999	6.4
2000	6.6
2001	6.7
2002	6.8
2003	7
2004	7.1
2005	7.2
2006	7.3
2007	7.5
2008	7.6
2009	7.7