Year	Percent
1960	54.28418
1961	54.05725
1962	53.75318
1963	54.18009
1964	53.70818
1965	53.44533
1966	53.96185
1967	53.71717
1968	52.73373
1969	52.92027
1970	53.27305
1971	54.12358
1972	53.8
1973	53.07309
1974	53.00438
1975	52.28866
1976	52.17551
1977	51.95191
1978	53.51876
1979	55.04187
1980	55.22769
1981	50.35979
1982	49.76601
1983	47.4392
1984	46.48687
1985	49.01545
1986	48.78296
1987	49.24044
1988	49.96356
1989	50.59782
1990	52.26737
1991	52.61525
1992	51.19087
1993	52.54351
1994	52.61459
1995	52.009
1996	52.74833
1997	52.46249
1998	52.7106
1999	54.03439
2000	55.69593
2001	56.52454
2002	57.27183
2003	57.59218
2004	57.11358
2005	57.60188
2006	57.97227
2007	57.76125
2008	59.18407
2009	60.84498
2010	61.80236
2011	63.11118
2012	64.14413
2013	65.6929
2014	66.3758
2015	67.39228
2016	67.65803
2017	68.2539