Year	Years
1970	1.6
1971	1.6
1972	1.7
1973	1.8
1974	1.8
1975	1.9
1976	2
1977	2
1978	2.1
1979	2.2
1980	2.2
1981	2.3
1982	2.4
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
1992	3.1
1993	3.2
1994	3.3
1995	3.3
1996	3.4
1997	3.5
1998	3.6
1999	3.7
2000	3.7
2001	3.8
2002	3.9
2003	4
2004	4.1
2005	4.2
2006	4.2
2007	4.3
2008	4.4
2009	4.5