Year	Percent
1960	19.91384
1961	19.89453
1962	19.36765
1963	20.62367
1964	20.08144
1965	18.69634
1966	19.02157
1967	22.08831
1968	22.7191
1969	22.44426
1970	22.25057
1971	24.24406
1972	23.31426
1973	26.2005
1974	22.97752
1975	25.94075
1976	26.46718
1977	21.3031
1978	21.45053
1979	20.66
1980	19.81093
1981	18.76586
1982	18.84452
1983	20.35731
1984	19.96613
1985	19.22958
1986	20.72949
1987	22.54788
1988	21.5124
1989	22.12623
1990	20.36915
1991	19.25549
1992	20.25968
1993	19.85311
1994	20.4112
1995	19.94489
1996	18.49222
1997	19.68278
1998	17.85594
1999	23.44586
2000	20.20465
2001	17.25903
2002	16.26176
2003	19.63226
2004	19.3934
2005	16.25808
2006	16.39393
2007	17.3726
2008	14.93044
2009	17.59685
2010	21.01561
2011	24.7024
2012	22.37308
2013	18.76144
2014	19.80378
2015	18.99623
2016	20.83394
2017	21.44817