Year	Percent
1960	34.181
1961	34.511
1962	34.844
1963	35.178
1964	35.515
1965	35.851
1966	36.19
1967	36.53
1968	36.871
1969	37.214
1970	37.466
1971	37.609
1972	37.753
1973	37.897
1974	38.042
1975	38.186
1976	38.331
1977	38.476
1978	38.621
1979	38.677
1980	38.627
1981	38.577
1982	38.527
1983	38.477
1984	38.427
1985	38.377
1986	38.327
1987	38.277
1988	38.227
1989	38.065
1990	37.777
1991	37.49
1992	37.203
1993	36.918
1994	36.633
1995	36.349
1996	36.066
1997	35.785
1998	35.504
1999	35.3
2000	35.298
2001	35.296
2002	35.295
2003	35.293
2004	35.292
2005	35.29
2006	35.289
2007	35.287
2008	35.285
2009	35.284
2010	35.306
2011	35.352
2012	35.423
2013	35.517
2014	35.635
2015	35.777
2016	35.944
2017	36.135