Year	Percent
1960	13.6
1961	13.747
1962	13.895
1963	14.045
1964	14.197
1965	14.349
1966	14.503
1967	14.658
1968	14.815
1969	14.972
1970	15.132
1971	15.293
1972	15.455
1973	15.618
1974	15.783
1975	15.95
1976	16.118
1977	16.287
1978	16.457
1979	16.77
1980	17.804
1981	18.883
1982	20.013
1983	21.194
1984	22.426
1985	23.704
1986	25.034
1987	26.413
1988	27.841
1989	29.312
1990	30.83
1991	32.39
1992	33.271
1993	33.635
1994	34.002
1995	34.371
1996	34.742
1997	35.114
1998	35.488
1999	35.865
2000	36.243
2001	36.622
2002	37.004
2003	37.387
2004	37.772
2005	38.157
2006	38.545
2007	38.934
2008	39.325
2009	39.716
2010	40.111
2011	40.508
2012	40.908
2013	41.31
2014	41.715
2015	42.123
2016	42.533
2017	42.945