Year	% of total
1971	17.90026933637925
1972	21.09162537793467
1973	20.79463974878035
1974	23.745736240860722
1975	23.478234369723115
1976	24.278077771986577
1977	25.41870736391809
1978	23.270251410756952
1979	23.61061260177134
1980	21.479716376635313
1981	20.374921610717028
1982	19.505827581858746
1983	17.347283450489083
1984	15.590956898101837
1985	16.667478952652342
1986	16.947151407306045
1987	15.262474708977559
1988	16.767298527205295
1989	16.83558480750657
1990	16.59722455344643
1991	18.083849519744433
1992	17.148283742652904
1993	16.240988058358372
1994	15.761315381768396
1995	15.84969220635252
1996	15.28020262377975
1997	15.02388472689747
1998	15.908305456446978
1999	16.73617325470337
2000	17.306171923207028
2001	18.522126003416638
2002	18.65121783850227
2003	17.272011925991993
2004	17.4337352768589
2005	17.84692638399742
2006	18.118365737272462
2007	17.355737866164862
2008	17.992421271560886
2009	19.04133034302595
2010	20.04026220642463
2011	20.104894376335817
2012	20.02057082294894
2013	20.45391317735374
2014	21.288358278003574
2015	20.633011633253986