Year	Percent
1970	31.9553
1971	31.96954
1972	48.1106
1973	37.55431
1974	51.76217
1975	62.67301
1976	73.49465
1977	58.07012
1978	35.01484
1979	32.91964
1980	27.53816
1981	36.35064
1982	35.01556
1983	35.18533
1984	32.96491
1985	41.99267
1986	46.36464
1987	26.36115
1988	37.47483
1989	26.30475
1990	21.67716
1991	26.46803
1992	22.42511
1993	22.44852
1994	21.86759
1995	23.30317
1996	24.84894
1997	30.76131
1998	39.55766
1999	26.16858
2000	21.90206
2001	22.74547
2002	25.1071
2003	22.6391
2004	21.49605
2005	21.36453
2006	24.0452
2007	24.87869
2008	25.0899
2009	27.98143
2010	29.70119
2011	26.64188
2012	26.96629
2013	29.18014
2014	35.13901
2015	29.23032
2016	27.88246
2017	28.77501