Year	Percent
1960	6.89427
1961	7.72941
1962	11.99041
1963	9.85563
1964	11.97517
1965	11.47628
1966	9.0169
1967	10.7087
1968	14.1548
1969	12.50836
1970	11.34466
1971	8.22783
1972	4.6977
1973	8.71201
1974	7.37486
1975	6.14791
1976	9.91136
1977	11.52322
1978	11.5468
1979	11.20387
1980	14.43939
1981	17.15577
1982	17.36328
1983	16.56274
1984	16.48426
1985	15.83094
1986	16.17645
1987	15.47344
1988	15.73598
1989	16.12091
1990	16.45868
1991	16.89595
1992	17.30503
1993	17.94683
1994	18.40256
1995	19.1198
1996	20.72995
1997	21.81621
1998	22.12141
1999	22.72137
2000	23.80856
2001	24.17431
2002	24.34142
2003	24.67919
2004	24.99183
2005	25.83044
2006	26.14415
2007	26.1785
2008	26.20227
2009	26.20606
2010	26.24666
2011	27.42097
2012	28.26234
2013	28.38962
2014	28.57788
2015	28.88669
2016	29.65257
2017	30.51045