Year	% of total
1971	0.25236986094872005
1972	0.2290727334937059
1973	0.24351472398719673
1974	0.2651028186703185
1975	0.2802789601891785
1976	0.2942409288477485
1977	0.2971070131183476
1978	0.2901349903635775
1979	0.33973554610772694
1980	0.3836224576507681
1981	0.4330176751175565
1982	0.4816291072043962
1983	0.5380941748463095
1984	0.5077800849057063
1985	0.5011906457169215
1986	0.5285278684983398
1987	0.5308660735201022
1988	0.5163485589847975
1989	0.5075364613687552
1990	0.527466645970269
1991	0.556749829901585
1992	0.6184563898207106
1993	0.6034969877493845
1994	0.6427982534944222
1995	0.7165596822574467
1996	0.708703022816056
1997	0.7382860254474596
1998	0.7624053281511103
1999	0.832040233524962
2000	0.8753299288147511
2001	0.90521575568876
2002	0.9092061564770533
2003	0.9267985881180348
2004	0.9344743648950828
2005	0.9929548790760079
2006	1.0477646095996958
2007	1.144792711437272
2008	1.351324069052453
2009	1.6724321557931545
2010	1.9652866723129578
2011	2.236567270638957
2012	2.6302847605949102
2013	3.225046804230299
2014	3.6767653784125858
2015	4.290399569970219