Year	% of total
1971	24.815179288315324
1972	23.3244458834131
1973	21.710782440547906
1974	20.99424425344781
1975	20.83163995362101
1976	18.995603567184848
1977	19.51964367337765
1978	20.333383570330824
1979	19.420394691754783
1980	18.75408742124353
1981	19.101387336075593
1982	19.38378176465456
1983	19.40925824683876
1984	20.05072001225933
1985	21.152089858787413
1986	21.848345731504708
1987	22.372543892219507
1988	21.991932756978933
1989	22.576309939896095
1990	32.79265826154189
1991	33.839988845060134
1992	35.64794372459847
1993	36.36239669452952
1994	38.80529773219692
1995	39.00373861637949
1996	40.31702116507333
1997	40.67519530032009
1998	40.32812680602418
1999	40.95941379419894
2000	42.04875116204317
2001	42.63480177038694
2002	44.05392291005354
2003	46.41693453703839
2004	46.86882435060928
2005	48.80870474058182
2006	50.70510608524867
2007	52.65154501548924
2008	51.95173333688932
2009	52.480631575887806
2010	52.579176275295325
2011	54.961639887935156
2012	53.33687918591031
2013	53.91448583974968
2014	52.55562277276139
2015	51.371989594953526