Year	Years
1970	1.4
1971	1.5
1972	1.5
1973	1.6
1974	1.7
1975	1.7
1976	1.8
1977	1.9
1978	2
1979	2
1980	2.1
1981	2.2
1982	2.3
1983	2.4
1984	2.5
1985	2.6
1986	2.7
1987	2.7
1988	2.8
1989	2.9
1990	3
1991	3.1
1992	3.2
1993	3.3
1994	3.4
1995	3.5
1996	3.6
1997	3.7
1998	3.8
1999	3.9
2000	4
2001	4.1
2002	4.3
2003	4.4
2004	4.5
2005	4.6
2006	4.7
2007	4.9
2008	5
2009	5.1