Year	Percent
1969	36.61991
1970	34.14212
1971	33.51911
1972	33.23065
1973	32.43509
1974	34.5005
1975	31.5318
1976	31.51182
1977	30.5287
1978	29.55916
1979	29.65058
1980	30.17607
1981	30.65282
1982	29.73728
1983	29.24877
1984	30.43765
1985	30.63122
1986	28.30394
1987	26.90821
1988	27.02934
1989	26.91695
1990	26.71957
1991	26.35735
1992	25.38612
1993	24.94701
1994	24.50989
1995	24.45434
1996	24.07967
1997	23.12877
1998	22.64105
1999	21.95676
2000	22.16305
2001	22.14724
2002	21.36578
2003	21.04778
2004	21.03928
2005	21.35467
2006	21.48384
2007	21.45774
2008	21.60513
2009	20.44191
2010	19.90144
2011	20.18512
2012	20.03863
2013	19.32047
2014	18.4764
2015	18.24782
2016	17.95782
2017	17.52845