Year	Years
1970	2.7
1971	2.8
1972	2.9
1973	3
1974	3.2
1975	3.3
1976	3.4
1977	3.6
1978	3.7
1979	3.8
1980	4
1981	4.1
1982	4.3
1983	4.4
1984	4.6
1985	4.7
1986	4.9
1987	5.1
1988	5.2
1989	5.4
1990	5.6
1991	5.8
1992	5.9
1993	6.1
1994	6.3
1995	6.5
1996	6.7
1997	6.9
1998	7
1999	7.2
2000	7.4
2001	7.6
2002	7.8
2003	8
2004	8.2
2005	8.4
2006	8.6
2007	8.8
2008	9
2009	9.2