Year	% of total
1971	4.044905983867187
1972	4.671751578182313
1973	5.081888322777527
1974	6.042855690074432
1975	6.289633234739195
1976	5.989554707618339
1977	5.53041951133457
1978	5.147164842021312
1979	4.969478960437526
1980	4.596910954502398
1981	4.283654172357269
1982	4.199224480552913
1983	4.254427165358908
1984	4.361647418565301
1985	3.791594912875111
1986	3.7982048106364075
1987	3.881570071476602
1988	3.8207327837869594
1989	4.287364416943679
1990	19.321308495904947
1991	18.89044699591065
1992	18.217092177781545
1993	18.176745188359018
1994	17.464998025315552
1995	17.454134041908443
1996	18.50375311173906
1997	19.522327778855388
1998	19.643997015348923
1999	21.38027902143683
2000	21.783434615231805
2001	22.15131874012612
2002	22.827991685269275
2003	23.91409928937963
2004	24.864654155947456
2005	25.958122837291626
2006	26.132293468170086
2007	27.633736461091658
2008	28.537514739183038
2009	28.113978757037415
2010	28.659918102881775
2011	27.810449169812895
2012	25.525596109134746
2013	24.445844368056424
2014	24.303785981248787
2015	24.320428221397925