Year	% of total
1971	31.528723171505
1972	30.4868812310979
1973	30.0454578723209
1974	30.2684161292188
1975	30.4399178251943
1976	31.2749003510894
1977	31.8531870720879
1978	31.4363268482097
1979	30.3793712031841
1980	30.1494923944625
1981	29.1564521966529
1982	28.356894452167
1983	27.226881467
1984	27.6431750588446
1985	27.9195793843961
1986	28.2180477114859
1987	28.3654879811469
1988	27.7675161089683
1989	27.6044453093594
1990	44.205223828963
1991	44.0891938711786
1992	44.2196769921013
1993	45.2862412375863
1994	43.4029365162133
1995	42.3914503050411
1996	40.8945215349101
1997	38.2314790502047
1998	41.2274845656496
1999	40.6095581844927
2000	39.7596719383245
2001	40.2805611226039
2002	39.9107963824571
2003	39.5322209734193
2004	39.5047035560107
2005	39.0590712751081
2006	37.9656916946897
2007	38.549339410525
2008	36.4719646751769
2009	35.5626400957785
2010	35.6546072940797
2011	38.3061374791651
2012	40.0003352204266
2013	43.413917124321
2014	44.7849890443168
2015	45.1151207950367