Year	Percent
1970	32.47029
1971	31.58199
1972	30.37203
1973	29.06865
1974	26.26568
1975	24.24776
1976	25.37854
1977	25.01613
1978	25.31531
1979	27.23263
1980	27.01094
1981	24.43011
1982	22.98541
1983	23.76795
1984	23.53234
1985	22.94995
1986	22.96998
1987	22.38223
1988	23.10868
1989	24.07562
1990	24.6507
1991	25.58799
1992	24.95836
1993	23.61168
1994	23.83273
1995	23.7411
1996	22.73511
1997	22.78289
1998	23.33981
1999	23.47381
2000	23.92326
2001	22.31397
2002	19.9192
2003	19.6874
2004	19.12081
2005	18.81453
2006	19.77261
2007	20.74931
2008	20.86004
2009	18.06746
2010	19.6254
2011	21.07753
2012	19.30677
2013	19.51221
2014	19.5172
2015	19.14846
2016	19.19788
2017	19.78136