Year	Percent
1960	33.464
1961	33.953
1962	34.714
1963	35.483
1964	36.26
1965	37.043
1966	37.833
1967	38.63
1968	39.435
1969	40.243
1970	41.057
1971	42.009
1972	43.064
1973	44.122
1974	45.187
1975	46.257
1976	47.331
1977	48.405
1978	49.482
1979	50.56
1980	51.638
1981	52.31
1982	52.777
1983	53.244
1984	53.711
1985	54.176
1986	54.641
1987	55.105
1988	55.568
1989	56.03
1990	56.491
1991	56.78
1992	56.719
1993	56.659
1994	56.598
1995	56.537
1996	56.476
1997	56.416
1998	56.355
1999	56.294
2000	56.233
2001	56.163
2002	56.013
2003	55.863
2004	55.713
2005	55.563
2006	55.412
2007	55.261
2008	55.111
2009	54.96
2010	54.685
2011	54.42
2012	54.28
2013	54.14
2014	54
2015	53.889
2016	53.806
2017	53.751