Year	Years
1970	1.8
1971	1.9
1972	1.9
1973	2
1974	2.1
1975	2.2
1976	2.3
1977	2.4
1978	2.5
1979	2.6
1980	2.8
1981	2.9
1982	3
1983	3.1
1984	3.2
1985	3.4
1986	3.5
1987	3.6
1988	3.7
1989	3.9
1990	4
1991	4.1
1992	4.3
1993	4.4
1994	4.5
1995	4.7
1996	4.8
1997	5
1998	5.1
1999	5.2
2000	5.4
2001	5.5
2002	5.7
2003	5.8
2004	6
2005	6.1
2006	6.3
2007	6.4
2008	6.6
2009	6.7