Year	Percent
1960	12.608
1961	12.821
1962	13.082
1963	13.578
1964	14.092
1965	14.62
1966	15.165
1967	15.727
1968	16.306
1969	16.88
1970	17.362
1971	17.855
1972	18.36
1973	18.874
1974	19.4
1975	19.873
1976	20.356
1977	20.846
1978	21.345
1979	21.853
1980	22.371
1981	22.895
1982	23.429
1983	24.069
1984	24.736
1985	25.414
1986	26.105
1987	26.809
1988	27.524
1989	28.25
1990	28.988
1991	29.738
1992	30.499
1993	30.94
1994	31.335
1995	31.732
1996	32.132
1997	32.534
1998	32.939
1999	33.347
2000	33.758
2001	34.17
2002	34.585
2003	34.479
2004	34.294
2005	34.11
2006	33.926
2007	33.743
2008	33.56
2009	33.378
2010	33.196
2011	33.015
2012	32.834
2013	32.654
2014	32.504
2015	32.385
2016	32.296
2017	32.237