Year	% of total
1971	31.40137801093165
1972	30.03258919966508
1973	25.70193568386579
1974	26.96834091061886
1975	25.109781121867147
1976	24.48655142635548
1977	24.09884303320073
1978	21.50688864920533
1979	18.46718148321781
1980	16.059467725503353
1981	14.114993874965107
1982	12.203962308642046
1983	10.155087259619096
1984	9.198261411092261
1985	8.124851827050234
1986	7.578704954733209
1987	7.875073844638288
1988	7.336645966279567
1989	7.4832525670267
1990	7.172893178700863
1991	6.626846794041634
1992	5.711751053997301
1993	5.333107225822684
1994	5.266648961113935
1995	5.029626584398298
1996	5.246199120940254
1997	5.117982872616899
1998	4.759518099484797
1999	4.797879225118126
2000	4.374007380259502
2001	4.481296386751145
2002	3.638291181322152
2003	3.7972944029720903
2004	3.745751638618571
2005	4.307738892712518
2006	3.956892059773006
2007	3.9498590509172447
2008	3.168147975605297
2009	3.0960947520039963
2010	3.5351140367632463
2011	3.2376961655529684
2012	3.198796695368364
2013	3.378876141458355
2014	2.824991982650229
2015	2.4299258101879455