Year	Percent
1960	16.79301
1961	21.17339
1962	20.30128
1963	17.61742
1964	17.46597
1965	17.53329
1966	18.56621
1967	17.08503
1968	19.74392
1969	21.44246
1970	23.47734
1971	22.3601
1972	26.45688
1973	24.592
1974	36.89993
1975	31.43555
1976	31.32894
1977	37.36806
1978	26.09884
1979	27.48751
1980	23.8985
1981	31.71801
1982	29.18013
1983	35.18223
1984	33.98125
1985	27.83199
1986	24.55938
1987	21.24802
1988	22.29161
1989	24.46296
1990	25.41054
1991	23.09264
1992	22.34883
1993	20.2643
1994	31.75374
1995	30.86634
1996	26.98402
1997	27.31489
1998	27.69656
1999	28.05647
2000	27.92512
2001	28.72629
2002	28.54887
2003	26.62285
2004	27.13868
2005	27.0174
2006	25.62624
2007	25.44628
2008	26.12822
2009	24.39399
2010	24.87803
2011	26.35075
2012	27.90555
2013	28.34568
2014	28.12071
2015	29.44312
2016	28.97154
2017	27.03546