Year	Percent
1971	0.31006
1972	0.30862
1974	0.31771
1985	0.34871
1987	0.36956
1989	0.39734
1990	0.41645
1994	0.44732
1995	0.44839
1996	0.45809
1997	0.48756
1998	0.51048
1999	0.5381
2000	0.56337
2001	0.58205
2002	0.59577
2003	0.60255
2004	0.60038
2005	0.61402
2006	0.6174
2007	0.64569
2008	0.6401
2009	0.6404
2010	0.66209
2011	0.68197
2012	0.69725
2013	0.70062
2014	0.69982
2015	0.70484
2016	0.70558