Year	Years
1970	2.7
1971	2.8
1972	2.9
1973	3
1974	3.2
1975	3.3
1976	3.4
1977	3.6
1978	3.7
1979	3.8
1980	3.9
1981	4.1
1982	4.2
1983	4.4
1984	4.5
1985	4.6
1986	4.8
1987	4.9
1988	5.1
1989	5.2
1990	5.3
1991	5.5
1992	5.6
1993	5.8
1994	5.9
1995	6
1996	6.2
1997	6.3
1998	6.5
1999	6.6
2000	6.7
2001	6.9
2002	7
2003	7.1
2004	7.3
2005	7.4
2006	7.5
2007	7.7
2008	7.8
2009	7.9