Year	Years
1970	2.8
1971	2.9
1972	2.9
1973	3
1974	3.1
1975	3.2
1976	3.3
1977	3.4
1978	3.4
1979	3.5
1980	3.6
1981	3.7
1982	3.8
1983	3.9
1984	4
1985	4.1
1986	4.2
1987	4.2
1988	4.3
1989	4.4
1990	4.5
1991	4.6
1992	4.7
1993	4.8
1994	4.9
1995	5
1996	5.1
1997	5.2
1998	5.3
1999	5.5
2000	5.6
2001	5.7
2002	5.8
2003	5.9
2004	6
2005	6.1
2006	6.2
2007	6.3
2008	6.4
2009	6.6