Year	Percent
1960	19.67804
1961	21.35614
1962	19.60466
1963	19.25988
1964	18.6024
1965	20.02392
1966	19.89289
1967	16.91871
1968	18.91756
1969	22.3522
1970	20.54203
1971	21.11791
1972	21.17081
1973	23.24409
1974	25.38869
1975	26.83887
1976	23.11621
1977	22.06915
1978	22.97322
1979	22.76445
1980	23.34647
1981	23.08412
1982	21.09376
1983	16.68204
1984	15.74056
1985	19.20075
1986	19.09311
1987	22.30466
1988	22.71853
1989	26.90279
1990	20.16661
1991	19.7691
1992	18.9346
1993	20.84636
1994	22.99349
1995	17.29235
1996	17.26695
1997	17.76399
1998	18.16475
1999	17.38956
2000	18.90312
2001	18.74186
2002	17.44908
2003	16.85669
2004	17.91257
2005	17.20488
2006	17.81647
2007	19.81933
2008	21.61938
2009	18.79614
2010	21.80132
2011	21.82627
2012	21.41723
2013	21.69377
2014	20.54841
2015	17.41163
2016	15.38724
2017	15.50282