Year	Years
1970	2
1971	2
1972	2.1
1973	2.2
1974	2.3
1975	2.4
1976	2.4
1977	2.5
1978	2.6
1979	2.7
1980	2.8
1981	2.9
1982	3
1983	3.1
1984	3.2
1985	3.3
1986	3.4
1987	3.5
1988	3.6
1989	3.7
1990	3.8
1991	3.9
1992	4
1993	4.1
1994	4.2
1995	4.3
1996	4.4
1997	4.5
1998	4.6
1999	4.7
2000	4.8
2001	4.9
2002	5
2003	5.1
2004	5.2
2005	5.3
2006	5.4
2007	5.5
2008	5.6
2009	5.7