Year	Percent
1970	15.17002
1971	14.60014
1972	14.59302
1973	15.3974
1974	18.28703
1975	17.1576
1976	18.1213
1977	18.0293
1978	17.68514
1979	17.87354
1980	18.67232
1981	20.19009
1982	20.92728
1983	20.34709
1984	21.78519
1985	23.00195
1986	21.34555
1987	20.73979
1988	21.14796
1989	22.36081
1990	22.8985
1991	23.72984
1992	22.2637
1993	20.35441
1994	21.13501
1995	22.00645
1996	22.9086
1997	25.39197
1998	26.45338
1999	27.04123
2000	30.82954
2001	31.87068
2002	32.57404
2003	32.59148
2004	35.44842
2005	37.74045
2006	41.19035
2007	43.00991
2008	43.45987
2009	37.8022
2010	42.25037
2011	44.81817
2012	45.98254
2013	45.39788
2014	45.7043
2015	46.87484
2016	46.11924
2017	47.23585