Year	Percent
1960	21.76843
1961	22.25887
1962	22.83731
1963	23.48354
1964	24.15111
1965	24.80937
1966	25.41798
1967	26.02156
1968	26.68433
1969	27.43973
1970	28.25649
1971	28.9151
1972	29.23378
1973	29.6125
1974	30.02264
1975	30.26532
1976	30.32375
1977	30.45754
1978	30.59402
1979	30.74646
1980	30.98223
1981	31.12659
1982	30.98182
1983	30.84843
1984	30.74136
1985	30.66821
1986	30.6129
1987	30.55659
1988	30.49166
1989	30.3785
1990	30.10405
1991	29.80831
1992	29.69031
1993	29.62446
1994	29.58479
1995	29.55507
1996	29.53338
1997	29.49587
1998	29.44062
1999	29.43569
2000	29.42338
2001	29.34228
2002	29.21779
2003	29.13107
2004	29.04205
2005	28.94007
2006	28.83638
2007	28.74615
2008	28.65313
2009	28.56115
2010	28.50766
2011	28.53314
2012	28.67104
2013	28.86276
2014	29.03864
2015	29.21347
2016	29.31803
2017	29.34319