Year	% of total
1971	6.538146406337504
1972	5.914001847441626
1973	5.576074560026725
1974	5.495118718467075
1975	5.581273282188772
1976	5.316019120170603
1977	6.245728559414231
1978	6.345519550492094
1979	7.373179655858431
1980	8.2970412090026
1981	8.787401303383062
1982	8.869121530251778
1983	9.027292636766727
1984	8.998974275234671
1985	9.5357705771711
1986	9.184443950856663
1987	10.295668652802282
1988	10.668891827035797
1989	11.49958590020717
1990	32.164812789524795
1991	30.843103299325296
1992	30.92640821443695
1993	29.784312404839618
1994	28.36032873434896
1995	28.72459468172307
1996	28.329097969360124
1997	29.454265475253845
1998	29.910462184019465
1999	29.779634078188288
2000	30.493660614791807
2001	31.02647629133457
2002	32.44370753389846
2003	35.3720285269036
2004	35.839812125777904
2005	33.721041947383505
2006	32.36440522732176
2007	31.59290246014834
2008	30.58141354835879
2009	32.04573573207732
2010	33.813535146157456
2011	33.42923720207002
2012	32.94423285384537
2013	32.62650755839621
2014	33.28761441738753
2015	33.85550036732175