Year	% of total
1971	17.91398572049438
1972	21.107787192590067
1973	20.81057399317468
1974	23.76393181285217
1975	23.49622496388171
1976	24.296681259679414
1977	25.438184878326503
1978	23.288082634455755
1979	23.628704632989074
1980	21.496175572582015
1981	20.390534238989634
1982	19.520774252133293
1983	17.360576099816853
1984	15.60290373250518
1985	16.68025069028653
1986	16.96013744868854
1987	15.27416983834807
1988	16.78014675327667
1989	16.848485359095115
1990	16.60994245745906
1991	18.097706575279854
1992	17.161423905088984
1993	16.253432990131834
1994	15.773392756241018
1995	15.861837301061637
1996	15.2919113374648
1997	15.035397032659088
1998	15.920495464550271
1999	16.748997630508295
2000	17.31943307013173
2001	18.536318895719976
2002	18.665509649607497
2003	17.2852468972411
2004	17.44709417136074
2005	17.860601893173147
2006	18.132249241443326
2007	17.36903699383164
2008	18.006208268654873
2009	19.055921084435496
2010	20.05561839626882
2011	20.12030009167329
2012	20.03591192393752
2013	20.469586334250234
2014	21.30467084257089
2015	20.648822027371292