Year	Percent
1960	7.10041
1961	8.12124
1962	9.28357
1963	9.31115
1964	9.26666
1965	9.48485
1966	12.45271
1967	8.42954
1968	7.9776
1969	9.35766
1970	10.77238
1971	12.36784
1972	14.21371
1973	15.51477
1974	12.27056
1975	19.24653
1976	22.26365
1977	19.64259
1978	22.53065
1979	28.54115
1980	24.58422
1981	24.15582
1982	20.97057
1983	20.82492
1984	22.90533
1985	20.68595
1986	20.00042
1987	20.80292
1988	18.29988
1989	16.59971
1990	14.81109
1991	14.05216
1992	16.75508
1993	15.64211
1994	16.53196
1995	17.68076
1996	18.5413
1997	16.82414
1998	17.78171
1999	15.90005
2000	17.79944
2001	16.92293
2002	15.90795
2003	15.19787
2004	17.36427
2005	16.59529
2006	16.41246
2007	17.42843
2008	17.73013
2009	20.32151
2010	22.19825
2011	20.90237
2012	21.85891
2013	22.63551
2014	21.01977
2015	18.18885
2016	16.16754
2017	16.62374