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	4.2
1982	4.3
1983	4.4
1984	4.5
1985	4.7
1986	4.8
1987	4.9
1988	5
1989	5.2
1990	5.3
1991	5.4
1992	5.5
1993	5.6
1994	5.7
1995	5.9
1996	6
1997	6.1
1998	6.2
1999	6.3
2000	6.5
2001	6.6
2002	6.7
2003	6.8
2004	6.9
2005	7
2006	7.1
2007	7.3
2008	7.4
2009	7.5