Year	Percent
1960	17.32746
1961	17.17452
1962	17.06102
1963	16.95279
1964	17.89264
1965	19.34884
1966	19.59087
1967	20.42553
1968	21.46032
1969	22.08904
1970	21.93384
1971	22.72165
1972	23.72881
1973	23.70879
1974	22.54942
1975	22.55662
1976	23.90671
1977	23.38351
1978	22.91548
1979	25.64343
1980	23.49444
1981	26.69409
1982	27.78254
1983	26.62918
1984	25.62338
1985	26.36606
1986	30.9879
1987	31.39374
1988	32.82056
1989	32.97471
1990	31.06107
1991	31.54085
1992	31.17906
1993	29.82298
1994	32.08109
1995	32.02013
1996	30.61805
1997	30.77986
1998	31.12813
1999	28.42469
2000	26.90259
2001	25.59681
2002	24.61117
2003	24.83314
2004	25.12998
2005	25.3019
2006	26.10879
2007	26.45674
2008	26.23655
2009	24.05599
2010	24.60176
2011	26.93581
2012	26.77414
2013	27.69614
2014	28.19517
2015	27.90429
2016	28.1833
2017	29.18801