Year	Years
1970	4
1971	4.1
1972	4.2
1973	4.3
1974	4.4
1975	4.5
1976	4.6
1977	4.8
1978	4.9
1979	5
1980	5.1
1981	5.2
1982	5.3
1983	5.4
1984	5.6
1985	5.7
1986	5.8
1987	5.9
1988	6.1
1989	6.2
1990	6.3
1991	6.5
1992	6.6
1993	6.7
1994	6.8
1995	7
1996	7.1
1997	7.2
1998	7.3
1999	7.4
2000	7.6
2001	7.7
2002	7.8
2003	7.9
2004	8
2005	8.1
2006	8.3
2007	8.4
2008	8.5
2009	8.6