Year	Years
1970	2.1
1971	2.2
1972	2.3
1973	2.4
1974	2.4
1975	2.5
1976	2.6
1977	2.7
1978	2.8
1979	2.9
1980	2.9
1981	3
1982	3.1
1983	3.2
1984	3.3
1985	3.3
1986	3.4
1987	3.5
1988	3.6
1989	3.7
1990	3.7
1991	3.8
1992	3.9
1993	4
1994	4.1
1995	4.1
1996	4.2
1997	4.3
1998	4.4
1999	4.5
2000	4.6
2001	4.6
2002	4.7
2003	4.8
2004	4.9
2005	5
2006	5.1
2007	5.2
2008	5.3
2009	5.4