Year	% of total
1971	12.14951045806989
1972	16.11051049443663
1973	15.49131591485971
1974	18.607782444051907
1975	18.495371368906003
1976	19.350048296241642
1977	20.83924935938937
1978	19.522333578736077
1979	18.855259191906157
1980	16.429487168525615
1981	15.533317164555665
1982	14.802584407411187
1983	14.344457313977351
1984	12.87710811039848
1985	12.921480962763662
1986	12.623344744364978
1987	11.009289480925805
1988	12.406019739446691
1989	12.261746991584891
1990	11.942703876125528
1991	12.8979780212612
1992	11.71400189797207
1993	11.131000507977088
1994	10.956611210440961
1995	10.969467043178803
1996	10.441763018087181
1997	10.182238191890745
1998	12.29493561229891
1999	12.688263728390591
2000	13.086301410551233
2001	14.547247931042184
2002	15.062842537734259
2003	13.928695301180325
2004	13.895481808982444
2005	14.630335075693859
2006	15.549263236241165
2007	15.244184880881118
2008	15.221488869071983
2009	16.50146218502922
2010	17.797542909822884
2011	17.90911011235687
2012	18.061799664256615
2013	18.69779864088079
2014	19.695138973812618
2015	19.541425303809724