Year	Percent
1960	6.12391
1961	4.58349
1962	3.51782
1963	3.82136
1964	5.13734
1965	7.26756
1966	6.84654
1967	7.66582
1968	9.33464
1969	11.77322
1970	15.17658
1971	17.7858
1972	14.3844
1973	15.66198
1974	14.31676
1975	14.52825
1976	12.99125
1977	16.1789
1978	16.52509
1979	17.51638
1980	15.81651
1981	15.9305
1982	17.77104
1983	13.76429
1984	19.57938
1985	23.65262
1986	16.67592
1987	15.26753
1988	13.80374
1989	13.49543
1990	19.1813
1991	21.00249
1992	23.23915
1993	22.46438
1994	28.32702
1995	27.36894
1996	26.49881
1997	27.17613
1998	27.09294
1999	23.68818
2000	21.14039
2001	20.34916
2002	20.18788
2003	18.98405
2004	18.81855
2005	18.93204
2006	18.52802
2007	22.58651
2008	22.85464
2009	20.37181
2010	23.78773
2011	21.38208
2012	24.59335
2013	28.15146
2014	32.07626
2015	28.22097
2016	28.70624
2017	30.93187