Year	% of total
1971	22.730634682308388
1972	21.68005176000888
1973	20.26690875149973
1974	19.80036925293676
1975	19.686295072487184
1976	18.489936425798774
1977	18.756304334511597
1978	19.147901892671758
1979	18.394053136202174
1980	17.950238043303035
1981	18.32347782997609
1982	18.687357508954914
1983	18.72219905503903
1984	19.35864593673656
1985	20.661036728370735
1986	21.44219631321379
1987	22.270842532905274
1988	21.90838668318348
1989	22.454967454392413
1990	32.625005596250325
1991	33.587038549364095
1992	35.09401339256863
1993	35.574407756388894
1994	37.18705585582461
1995	37.55452397565138
1996	38.37269809679318
1997	38.63921201317574
1998	38.19485364241281
1999	38.840824323220396
2000	39.85851868912931
2001	40.36912090948631
2002	41.366122923467394
2003	42.87222691592607
2004	43.00110710589038
2005	44.45097332246484
2006	46.179774487360945
2007	47.73664373224466
2008	47.46280876698789
2009	47.79445644361641
2010	47.98730896559145
2011	50.048876535512065
2012	49.477490443528055
2013	50.2069973810644
2014	49.46368131942432
2015	48.886106283784855