Year	Percent
1960	15.95623
1961	16.36849
1962	16.8251
1963	17.30196
1964	17.77258
1965	18.23743
1966	18.72705
1967	19.19021
1968	19.65078
1969	20.1854
1970	20.70976
1971	20.97936
1972	21.02435
1973	21.06898
1974	21.11716
1975	21.16832
1976	21.22565
1977	21.28873
1978	21.36702
1979	21.46352
1980	21.57829
1981	21.64913
1982	21.61696
1983	21.60936
1984	21.61753
1985	21.63694
1986	21.67037
1987	21.71249
1988	21.76519
1989	21.82989
1990	21.90704
1991	21.99001
1992	22.1
1993	22.20386
1994	22.31829
1995	22.44142
1996	22.57147
1997	22.70136
1998	22.83385
1999	22.96938
2000	23.0967
2001	23.16713
2002	23.13317
2003	23.03902
2004	22.96475
2005	22.89812
2006	22.8319
2007	22.72931
2008	22.68557
2009	22.80227
2010	23.01772
2011	23.25964
2012	23.57291
2013	23.98346
2014	24.39417
2015	24.75705
2016	25.08466
2017	25.39003