Year	Percent
1960	24.71156
1961	27.0997
1962	30.78181
1963	26.98891
1964	29.41983
1965	29.62986
1966	29.64519
1967	29.31362
1968	27.90605
1969	29.20234
1970	30.31858
1971	30.45516
1972	31.6774
1973	31.25223
1974	35.8556
1975	36.13118
1976	37.28249
1977	38.38664
1978	38.50368
1979	39.83066
1980	41.79196
1981	41.05672
1982	38.56669
1983	38.95189
1984	39.2334
1985	39.23426
1986	39.23364
1987	38.52747
1988	38.35661
1989	39.80405
1990	42.19817
1991	42.10608
1992	41.14454
1993	40.08525
1994	40.04144
1995	41.40251
1996	43.5272
1997	44.57354
1998	43.8763
1999	46.45971
2000	48.32001
2001	46.20355
2002	45.11576
2003	46.57842
2004	48.52979
2005	45.92699
2006	46.09585
2007	42.21521
2008	42.91611
2009	38.46347
2010	40.49893
2011	39.82427
2012	40.135
2013	39.88537
2014	39.92451
2015	39.09934
2016	38.33691
2017	38.82772