Year	% of total
1971	26.736327967496738
1972	24.70506820125825
1973	24.606836131496507
1974	23.52263779653054
1975	23.851934392461395
1976	25.049130056715217
1977	24.54780956022739
1978	24.705511490440234
1979	24.860428333129764
1980	25.78159414848738
1981	25.071305494757922
1982	25.840711390035963
1983	25.690572552740655
1984	23.42665621076489
1985	24.09122653115547
1986	24.229327765091817
1987	23.74571774265872
1988	22.91919649725013
1989	23.107723402736625
1990	31.443515512718218
1991	31.240946379963948
1992	30.803051867192664
1993	29.686264795063405
1994	30.285278445409276
1995	29.87699513342469
1996	29.30980867970026
1997	28.114750720184457
1998	28.03617769966681
1999	26.85916235556776
2000	27.68840250726897
2001	27.315712428981588
2002	27.3138062596217
2003	27.885653413181373
2004	26.58583261570634
2005	26.081970170372916
2006	26.5868895629594
2007	26.464402802671827
2008	25.39536804104602
2009	24.07720632965061
2010	23.465703423127916
2011	24.437763394803326
2012	25.285240865834535
2013	24.940242078041106
2014	23.789202589045896
2015	23.204474326869924