Year	Percent
1960	15.83278
1961	15.52448
1962	12.79813
1963	14.76883
1964	13.22659
1965	12.65381
1966	13.40449
1967	12.90648
1968	13.09852
1969	14.0641
1970	13.96934
1971	12.04259
1972	13.16157
1973	15.38797
1974	19.3038
1975	26.57162
1976	20.3069
1977	22.02143
1978	23.65739
1979	25.54807
1980	26.44875
1981	26.21654
1982	21.25209
1983	21.39706
1984	24.46084
1985	24.33277
1986	24.73354
1987	25.84909
1988	25.98107
1989	29.2729
1990	29.25111
1991	26.79328
1992	27.29226
1993	27.72838
1994	25.75179
1995	26.3409
1996	28.12243
1997	28.47616
1998	28.80003
1999	26.58114
2000	28.78412
2001	30.7445
2002	30.58979
2003	30.65724
2004	29.93129
2005	31.45694
2006	29.30354
2007	31.33841
2008	39.36561
2009	29.29643
2010	31.3186
2011	34.44371
2012	34.14999
2013	32.774
2014	32.1566
2015	29.58617
2016	27.32915
2017	26.99147