Year	Years
1970	2.6
1971	2.7
1972	2.8
1973	2.9
1974	2.9
1975	3
1976	3.1
1977	3.2
1978	3.3
1979	3.4
1980	3.5
1981	3.6
1982	3.7
1983	3.9
1984	4
1985	4.1
1986	4.2
1987	4.3
1988	4.4
1989	4.5
1990	4.6
1991	4.7
1992	4.9
1993	5
1994	5.1
1995	5.2
1996	5.3
1997	5.4
1998	5.6
1999	5.7
2000	5.8
2001	5.9
2002	6
2003	6.2
2004	6.3
2005	6.4
2006	6.5
2007	6.6
2008	6.8
2009	6.9