Year	% of total
1971	18.034788187631154
1972	19.817655472527633
1973	21.169984178909935
1974	19.841418837453933
1975	18.738173301361417
1976	19.533430360871904
1977	19.123893487148695
1978	18.93743349120067
1979	17.530393602591293
1980	15.78110994237219
1981	14.44471936564148
1982	12.895667056492648
1983	11.969669167075416
1984	10.861070063621742
1985	9.15835769363592
1986	9.251655985361094
1987	8.84142214881353
1988	9.10063500235257
1989	9.400939488076878
1990	10.202052481354045
1991	10.036017450596434
1992	9.71832539879453
1993	9.102919568428755
1994	8.837433207041267
1995	8.484872213132777
1996	8.090684594266605
1997	8.03368243072554
1998	8.204241817104151
1999	7.783791309737098
2000	7.298836176189058
2001	7.046548582491834
2002	6.727198342338209
2003	6.575714025038279
2004	6.167517728824524
2005	5.748395474188458
2006	5.064642323486
2007	4.868313128891896
2008	4.595082443011914
2009	4.181064654550936
2010	3.810264981100281
2011	4.0187985282466805
2012	4.153661732644955
2013	3.78169654687084
2014	3.5458545543130895
2015	3.2777917464509256