Year	Years
1970	3
1971	3.1
1972	3.2
1973	3.3
1974	3.4
1975	3.5
1976	3.6
1977	3.7
1978	3.8
1979	3.9
1980	4
1981	4.1
1982	4.2
1983	4.3
1984	4.4
1985	4.5
1986	4.7
1987	4.8
1988	4.9
1989	5
1990	5.1
1991	5.2
1992	5.4
1993	5.5
1994	5.6
1995	5.7
1996	5.8
1997	6
1998	6.1
1999	6.2
2000	6.3
2001	6.4
2002	6.6
2003	6.7
2004	6.8
2005	6.9
2006	7.1
2007	7.2
2008	7.3
2009	7.4