Year	% of total
1971	19.3282271452705
1972	20.3714284524398
1973	20.8328913172651
1974	20.6109071892456
1975	20.440691988654
1976	19.6066674518738
1977	19.0290612431657
1978	19.4469956838541
1979	20.504560769504
1980	20.7353781095049
1981	21.7290999389712
1982	22.5298273448315
1983	23.6607936713904
1984	23.2450481139386
1985	22.9660476992514
1986	22.6619980286263
1987	22.5197268997158
1988	23.1191254851745
1989	23.2835461494872
1990	24.5958627526361
1991	24.7116019772739
1992	24.7475142052427
1993	23.6809499597577
1994	25.5642546811308
1995	26.5757408923029
1996	28.0726696624339
1997	30.7346895322822
1998	27.7143289310522
1999	28.3395279935983
2000	29.3701854017559
2001	28.8518887898566
2002	29.2224685176231
2003	29.6003715519031
2004	29.6280256332268
2005	30.0725424730284
2006	31.1618451571046
2007	30.5744115718323
2008	32.6474633034425
2009	33.5506165501611
2010	33.4523260990448
2011	30.7854697752832
2012	29.2468500664533
2013	25.7170299745967
2014	24.2542741259537
2015	23.728560708569