Year	Years
1970	3
1971	3.1
1972	3.3
1973	3.4
1974	3.5
1975	3.7
1976	3.8
1977	3.9
1978	4.1
1979	4.2
1980	4.4
1981	4.5
1982	4.7
1983	4.8
1984	5
1985	5.1
1986	5.3
1987	5.4
1988	5.6
1989	5.7
1990	5.9
1991	6.1
1992	6.2
1993	6.4
1994	6.5
1995	6.7
1996	6.8
1997	7
1998	7.1
1999	7.3
2000	7.5
2001	7.6
2002	7.8
2003	7.9
2004	8.1
2005	8.2
2006	8.4
2007	8.5
2008	8.7
2009	8.8