Year	Percent
1960	35.111
1961	35.829
1962	36.555
1963	37.286
1964	38.025
1965	38.767
1966	39.516
1967	40.269
1968	41.028
1969	41.79
1970	42.556
1971	43.326
1972	44.1
1973	43.881
1974	43.662
1975	43.443
1976	43.224
1977	43.005
1978	42.787
1979	42.569
1980	42.351
1981	42.134
1982	41.917
1983	41.701
1984	42.012
1985	42.325
1986	42.639
1987	42.953
1988	43.269
1989	43.584
1990	43.9
1991	43.777
1992	43.653
1993	43.53
1994	43.407
1995	43.284
1996	43.161
1997	43.038
1998	42.916
1999	42.793
2000	42.67
2001	42.559
2002	42.447
2003	42.335
2004	42.223
2005	42.112
2006	42
2007	41.889
2008	41.777
2009	41.666
2010	41.555
2011	41.444
2012	41.332
2013	41.222
2014	41.111
2015	41
2016	40.91
2017	40.841