Year	Percent
1960	19.68856
1961	19.81912
1962	19.96296
1963	20.11714
1964	20.28587
1965	20.47389
1966	20.70013
1967	20.96293
1968	21.2454
1969	21.52418
1970	21.7783
1971	21.99205
1972	22.21513
1973	22.4685
1974	22.72817
1975	23.02083
1976	23.32185
1977	23.63482
1978	23.9761
1979	24.23211
1980	24.4018
1981	24.57718
1982	24.74784
1983	24.91165
1984	25.08336
1985	25.24942
1986	25.41418
1987	25.59604
1988	25.73735
1989	25.75269
1990	25.4859
1991	25.03828
1992	24.58069
1993	24.13362
1994	24.12025
1995	24.50714
1996	24.8656
1997	25.06044
1998	25.03887
1999	24.9664
2000	24.89188
2001	24.80681
2002	24.82353
2003	25.1406
2004	25.46601
2005	25.79396
2006	26.12435
2007	26.46345
2008	26.79836
2009	27.14203
2010	27.49478
2011	27.84983
2012	28.20698
2013	28.56602
2014	28.9345
2015	29.00447
2016	28.98021
2017	28.99026