Year	% of total
1971	64.6107296808363
1972	65.20646698717078
1973	64.71392646937238
1974	64.0844519771131
1975	63.49047516067963
1976	62.55705239372411
1977	62.83652184605097
1978	61.84461092613907
1979	62.75123560629707
1980	63.716798715591395
1981	63.48182140793502
1982	65.2911309159998
1983	66.33940835552617
1984	65.74067273759911
1985	66.29654613490646
1986	65.90649419540038
1987	66.9007001426937
1988	65.67200745618432
1989	67.05477578995381
1990	69.98338692750022
1991	70.50686711837811
1992	70.58145104015934
1993	70.6178491324895
1994	70.91223592027073
1995	71.54459518181258
1996	72.15977340105448
1997	72.82812199441038
1998	73.21293217356275
1999	74.22940294426508
2000	75.27333540538507
2001	76.54039315682212
2002	77.17478641325067
2003	77.55250976184702
2004	77.09372992072156
2005	76.64607483558841
2006	76.20675834549019
2007	77.56868887726769
2008	78.09533987049184
2009	77.88764731972351
2010	77.19263405278255
2011	76.60304160121116
2012	77.45190984154524
2013	76.89293294872421
2014	77.6123960768945
2015	78.00144980006432