Year	Percent
1960	19.68191
1961	20.19599
1962	20.41992
1963	21.35601
1964	21.72278
1965	23.03424
1966	22.85701
1967	23.08207
1968	23.98814
1969	23.92976
1970	24.20722
1971	25.45187
1972	26.1058
1973	26.6994
1974	27.57629
1975	27.37286
1976	27.4483
1977	26.16262
1978	26.06752
1979	26.45673
1980	26.96767
1981	26.62116
1982	25.73927
1983	28.72572
1984	29.97434
1985	29.47838
1986	28.388
1987	27.97794
1988	28.41575
1989	28.5521
1990	27.01432
1991	27.47679
1992	27.98088
1993	27.48293
1994	26.73742
1995	26.69694
1996	26.10231
1997	26.16766
1998	26.10864
1999	25.8276
2000	25.55236
2001	25.31142
2002	24.67014
2003	24.28068
2004	24.77485
2005	24.37623
2006	23.83503
2007	24.01304
2008	23.68996
2009	23.64502
2010	23.23968
2011	22.32011
2012	21.93474
2013	20.76092
2014	20.17244
2015	19.42181
2016	18.90863
2017	18.57043