Year	% of total
1971	25.79202153611418
1972	24.861831262166014
1973	23.821259248527937
1974	23.46936124460573
1975	23.462060894865168
1976	22.265441992610054
1977	22.389769718108976
1978	22.657266174205517
1979	21.85802244587584
1980	21.29097267378589
1981	20.77611894582649
1982	21.094608621929066
1983	21.169292281542358
1984	21.836157125319406
1985	23.281858055822784
1986	23.921123015316265
1987	24.654446846422676
1988	24.30024855701855
1989	24.793265402715665
1990	34.11323348454036
1991	34.819236099944135
1992	36.17467991077533
1993	36.60810004973642
1994	38.148906430661114
1995	38.516679285949714
1996	39.28705414565507
1997	39.42364537460344
1998	38.95542579430482
1999	39.54094282457401
2000	40.45631159064173
2001	40.76904716556398
2002	41.66964502061492
2003	43.17726815908764
2004	43.21497056726153
2005	44.50026164962509
2006	46.21346101477841
2007	47.680768198334476
2008	47.35570344771072
2009	47.60991541495425
2010	47.833506278262725
2011	49.88305820020696
2012	49.316184017526574
2013	50.01310305709349
2014	49.18521904806768
2015	48.66122393630172