Year	% of total
1971	17.096026294839916
1972	17.47024422374252
1973	17.56719609411225
1974	18.527306306531617
1975	18.2300206762434
1976	18.43568614394384
1977	18.646112095144428
1978	18.246513457702818
1979	18.520382319232695
1980	18.693275564246324
1981	19.06455837534482
1982	18.890329112072926
1983	18.570262763170298
1984	18.526852319792145
1985	18.35350668909118
1986	18.032522604157357
1987	17.524268389130853
1988	17.91726515698036
1989	17.492674302905016
1990	23.47280639846188
1991	23.858432892363588
1992	24.03449429291896
1993	25.043003047946705
1994	25.770797907708804
1995	25.69657783904855
1996	24.895076221982716
1997	24.64754741406388
1998	24.95524228158792
1999	24.140889774714385
2000	23.469220341900147
2001	23.127087003727762
2002	22.464222689654544
2003	21.28435734050739
2004	21.920133371880645
2005	21.991634547623278
2006	21.48544710842259
2007	20.601870305287072
2008	20.746677680184487
2009	20.694860991705504
2010	20.645479266973428
2011	19.51051368815327
2012	20.016720280415473
2013	19.800089843659265
2014	19.961082645317134
2015	19.61902579316151