Year	Percent
1960	26.7011
1961	26.94153
1962	27.25858
1963	27.57564
1964	27.89145
1965	28.19893
1966	28.50365
1967	28.80518
1968	29.10518
1969	29.39824
1970	29.68985
1971	29.90637
1972	30.09896
1973	30.28761
1974	30.48212
1975	30.68336
1976	30.89702
1977	31.11589
1978	31.33548
1979	31.54179
1980	31.73075
1981	31.89592
1982	31.71707
1983	31.46813
1984	31.24669
1985	31.06
1986	30.91272
1987	30.79856
1988	30.71475
1989	30.65218
1990	30.61039
1991	30.58987
1992	30.59217
1993	30.60735
1994	30.63162
1995	30.64848
1996	30.66147
1997	30.67387
1998	30.70936
1999	30.79177
2000	30.93551
2001	31.17046
2002	31.49035
2003	31.81644
2004	32.04634
2005	32.1228
2006	32.00122
2007	31.7593
2008	31.60512
2009	31.78107
2010	32.44587
2011	27.33092
2012	24.15477
2013	22.19565
2014	20.90777
2015	23.7168
2016	27.74529
2017	28.67488