Year	Percent
1970	28.05071
1971	26.09628
1972	28.72329
1973	31.24844
1974	29.63598
1975	19.44077
1976	24.76518
1977	30.10515
1978	31.60299
1979	30.58083
1980	34.00494
1981	35.82898
1982	35.35959
1983	29.30516
1984	23.04645
1985	21.39221
1986	23.16011
1987	27.36433
1988	30.45501
1989	28.27883
1990	27.70678
1991	25.72536
1992	24.93309
1993	22.13399
1994	22.93889
1995	24.18086
1996	24.38101
1997	26.51056
1998	28.30198
1999	29.01486
2000	28.76434
2001	28.14277
2002	25.9997
2003	23.63025
2004	24.17976
2005	23.65724
2006	23.2336
2007	23.07131
2008	23.56599
2009	20.79135
2010	21.0807
2011	18.59842
2012	15.71646
2013	14.63198
2014	15.30302
2015	15.8232
2016	15.48197
2017	16.32339