Year	% of total
1971	13.880475523685792
1972	13.676278578425304
1973	12.609429219651103
1974	13.151641600772956
1975	15.021757649134335
1976	16.32384476683009
1977	16.878510114119415
1978	18.211170988113665
1979	20.56361874247516
1980	22.09227965899523
1981	23.371212957697747
1982	23.815589560974317
1983	24.434978015197533
1984	29.54654134956607
1985	30.266823840544312
1986	30.085859465947635
1987	29.462590038352204
1988	29.612247721865042
1989	29.63093281010482
1990	30.708350677842724
1991	31.830313129064248
1992	34.32266836974823
1993	38.008972767653475
1994	37.72918842251319
1995	37.755019873284134
1996	38.696088070505255
1997	39.10679097254921
1998	39.79405687064096
1999	39.24505143227453
2000	39.59838544071357
2001	40.421880944975456
2002	40.52922251640569
2003	40.76425033385919
2004	41.33617132495949
2005	43.321349328667914
2006	44.24671273119093
2007	44.707153508738095
2008	45.4139605853777
2009	46.85055351891572
2010	46.8748947045561
2011	48.4221511507187
2012	50.03462637363144
2013	49.27610239571723
2014	50.82336128770799
2015	52.34488316102909