Year	% of total
1971	67.88297450490964
1972	64.4606247649522
1973	64.8656903305477
1974	63.003429585024705
1975	62.902981644394714
1976	61.93279195334652
1977	60.31989889747735
1978	62.05598853744674
1979	61.44110474236114
1980	63.43045804639102
1981	64.93183963198655
1982	65.67219382410308
1983	67.01666278319362
1984	67.31231259111668
1985	65.98892737263914
1986	64.0634372592474
1987	66.50288651044232
1988	63.72205298611179
1989	63.64558884318123
1990	64.78644620848083
1991	64.03396132014859
1992	64.61904171250983
1993	66.35346923104845
1994	65.51109729285123
1995	64.75491725000761
1996	65.4655788915636
1997	65.27569086987197
1998	63.902087150654474
1999	62.750445716956804
2000	62.55823147918965
2001	62.01063290138524
2002	60.757331033462734
2003	62.331599429382514
2004	61.2979087061949
2005	61.09102252696855
2006	60.42832030991434
2007	60.75403652222161
2008	59.918374630320606
2009	58.47364012720043
2010	57.4024406499563
2011	56.52994147061678
2012	55.76651587397174
2013	54.124153237462636
2014	52.06778802116606
2015	51.063006736298