Year	% of total
1971	17.900269336379253
1972	21.091625377934676
1973	20.79463974878036
1974	23.745736240860733
1975	23.478234369723122
1976	24.278077771986588
1977	25.4187073639181
1978	23.270251410756963
1979	23.61061260177135
1980	21.47971637663532
1981	20.37492161071704
1982	19.505827581858753
1983	17.34728345048909
1984	15.59095689810184
1985	16.667478952652345
1986	16.94715140730605
1987	15.262474708977564
1988	16.767298527205302
1989	16.835584807506574
1990	16.59722455344643
1991	18.083849519744437
1992	17.148283742652907
1993	16.240988058358376
1994	15.761315381768398
1995	15.849692206352522
1996	15.280202623779754
1997	15.023884726897471
1998	15.90830545644698
1999	16.736173254703374
2000	17.306171923207035
2001	18.522126003416638
2002	18.651217838502273
2003	17.272011925991997
2004	17.433735276858904
2005	17.84692638399742
2006	18.118365737272466
2007	17.355737866164866
2008	17.992421271560886
2009	19.041330343025955
2010	20.04026220642463
2011	20.10489437633582
2012	20.020570822948944
2013	20.453913177353744
2014	21.288358278003578
2015	20.63301163325399