Year	Years
1970	2.4
1971	2.5
1972	2.5
1973	2.6
1974	2.7
1975	2.8
1976	2.9
1977	3
1978	3.1
1979	3.2
1980	3.3
1981	3.4
1982	3.5
1983	3.6
1984	3.7
1985	3.8
1986	3.9
1987	4
1988	4.1
1989	4.2
1990	4.3
1991	4.4
1992	4.5
1993	4.6
1994	4.7
1995	4.8
1996	5
1997	5.1
1998	5.2
1999	5.3
2000	5.4
2001	5.5
2002	5.7
2003	5.8
2004	5.9
2005	6
2006	6.2
2007	6.3
2008	6.4
2009	6.5