Year	% of total
1971	23.00650897260131
1972	22.008534741728404
1973	20.468926585334827
1974	20.028496536744605
1975	19.820705735917443
1976	18.6196599978259
1977	18.882275192286375
1978	19.264042296680074
1979	18.480133020650193
1980	17.992842622218085
1981	18.37700404520666
1982	18.770598582584213
1983	18.795538842113505
1984	19.4651884880104
1985	20.787024404510525
1986	21.607678198609253
1987	22.414324578912485
1988	22.056077270631313
1989	22.571738158148342
1990	33.00476393263293
1991	33.98219516749958
1992	35.51301416410851
1993	36.02997209926592
1994	37.685938619313895
1995	38.08815743950749
1996	38.92061488758184
1997	39.18263499241213
1998	38.797468267939
1999	39.451942472773744
2000	40.49071451360396
2001	41.03269974077306
2002	42.05153519348975
2003	43.56237240026667
2004	43.695961262155734
2005	45.179933073383786
2006	46.93404795485747
2007	48.52221775341358
2008	48.23373897136168
2009	48.579914601556766
2010	48.768639792300185
2011	50.82412064377855
2012	50.20206744896147
2013	50.9017832241902
2014	50.133962427808555
2015	49.52476270478895