Year	Percent
1960	11.19816
1961	11.66564
1962	11.95162
1963	12.24191
1964	12.53653
1965	12.83223
1966	13.13295
1967	13.43858
1968	13.74859
1969	14.05812
1970	14.33915
1971	14.61739
1972	14.89651
1973	15.18404
1974	15.49659
1975	15.84159
1976	16.18841
1977	16.42252
1978	16.66782
1979	16.90417
1980	17.11911
1981	17.30684
1982	17.47515
1983	17.62859
1984	17.77613
1985	17.92097
1986	18.06812
1987	18.21632
1988	18.35103
1989	18.33395
1990	18.31873
1991	18.30356
1992	18.29191
1993	18.28988
1994	18.30957
1995	18.35554
1996	18.42991
1997	18.52506
1998	18.63352
1999	18.74195
2000	18.84217
2001	18.92929
2002	19.00739
2003	19.0086
2004	18.99514
2005	18.9757
2006	18.9533
2007	18.92615
2008	18.89209
2009	18.84568
2010	18.7877
2011	18.7176
2012	18.63961
2013	18.55799
2014	18.48207
2015	18.41599
2016	18.36089
2017	18.31551