Year	Years
1970	1.6
1971	1.7
1972	1.8
1973	1.9
1974	2
1975	2.1
1976	2.1
1977	2.2
1978	2.4
1979	2.5
1980	2.6
1981	2.7
1982	2.8
1983	3
1984	3.1
1985	3.2
1986	3.4
1987	3.5
1988	3.7
1989	3.8
1990	3.9
1991	4.1
1992	4.2
1993	4.4
1994	4.5
1995	4.7
1996	4.8
1997	5
1998	5.1
1999	5.3
2000	5.4
2001	5.6
2002	5.7
2003	5.9
2004	6
2005	6.2
2006	6.3
2007	6.4
2008	6.6
2009	6.7