Year	Percent
1970	18.29826
1971	19.61284
1972	22.2108
1973	19.8339
1974	25.70957
1975	26.64953
1976	24.5419
1977	27.92478
1978	32.46115
1979	28.44678
1980	28.56101
1981	36.54941
1982	46.81643
1983	54.50972
1984	54.64646
1985	52.86771
1986	53.49679
1987	54.07276
1988	51.57607
1989	51.08693
1990	47.76568
1991	43.97177
1992	40.13537
1993	29.19839
1994	27.07029
1995	27.32112
1996	28.17792
1997	29.19715
1998	27.94895
1999	30.88854
2000	35.20711
2001	39.45192
2002	41.89942
2003	57.14942
2004	57.80055
2005	58.3568
2006	56.06765
2007	50.03305
2008	50.61635
2009	51.57012
2010	52.96789
2011	53.26414
2012	56.86694
2013	57.96234
2014	60.50198
2015	59.74394
2016	60.31929