Year	% of total
1960	0.1669562424936483
1961	0.2510427337138185
1962	0.3361969311898309
1963	0.5004107037691977
1964	0.6476656011826937
1965	0.9543522469066293
1966	1.2125416322778881
1967	1.3646312399021248
1968	1.5868901381837688
1969	1.7324436372122696
1970	1.976166350945021
1971	2.6426604391563
1972	3.338259259178835
1973	4.118781445978012
1974	5.232026352099427
1975	7.100136788453994
1976	7.627402420021615
1977	9.112208768339269
1978	10.230612407601326
1979	10.176628204405018
1980	10.772791676364893
1981	12.537992200617989
1982	13.56675992694665
1983	14.791922896082813
1984	16.900629346730568
1985	19.413556677667884
1986	20.80036494980093
1987	21.54270910047323
1988	22.330890542525708
1989	21.823328781975448
1990	22.466680018100142
1991	23.1506501252955
1992	23.180013407420113
1993	23.38329332463902
1994	23.51739381682129
1995	23.68601146508406
1996	23.70013943060143
1997	23.073276902439442
1998	23.100951888874217
1999	23.35069327028509
2000	22.801842560325813
2001	23.310535754153737
2002	22.697593954315032
2003	21.923901482957383
2004	22.29391545501538
2005	21.936093769841357
2006	21.838717233154977
2007	20.737029067066228
2008	20.738695153832918
2009	21.120243934785936
2010	20.52723558757815
2011	18.8538121488406
2012	17.640343044280012
2013	17.657946982632648
2014	17.80769344803382
2015	17.15014225429176