Year	Percent
1960	21.383
1961	22.141
1962	22.962
1963	23.805
1964	24.669
1965	25.553
1966	26.328
1967	26.723
1968	27.122
1969	27.524
1970	27.951
1971	28.417
1972	28.888
1973	29.362
1974	29.842
1975	30.326
1976	30.815
1977	31.307
1978	31.355
1979	31.27
1980	31.186
1981	31.101
1982	31.017
1983	30.933
1984	30.849
1985	30.766
1986	30.682
1987	30.598
1988	30.515
1989	30.432
1990	30.594
1991	31.154
1992	31.872
1993	32.596
1994	33.33
1995	34.071
1996	34.822
1997	35.354
1998	35.654
1999	35.86
2000	36.337
2001	37.175
2002	38.022
2003	38.714
2004	39.247
2005	39.781
2006	40.147
2007	40.341
2008	40.536
2009	40.731
2010	40.926
2011	41.088
2012	41.225
2013	41.363
2014	41.501
2015	41.638
2016	41.777
2017	41.914