Year	Percent
1970	0.16697
1971	0.14991
1972	0.22734
1973	0.44066
1974	0.41949
1975	0.33804
1976	0.33967
1977	0.22803
1978	0.23392
1979	0.27469
1980	0.21576
1981	0.40148
1982	0.40017
1983	0.29051
1984	0.45245
1985	0.46037
1986	0.69277
1987	0.79361
1988	1.15364
1989	1.51399
1990	1.62074
1991	0.88266
1992	0.4427
1993	0.31236
1994	0.36928
1995	0.4153
1996	0.54622
1997	0.54334
1998	0.49114
1999	0.53182
2000	0.92127
2001	0.82863
2002	0.75093
2003	0.77524
2004	0.84346
2005	1.08645
2006	1.28361
2007	1.61651
2008	2.25577
2009	1.40837
2010	1.39746
2011	1.89746
2012	1.89631
2013	3.01965
2014	2.84356
2015	3.14939
2016	3.51418
2017	3.45987