Year	Percent
1971	0.34608
1972	0.35685
1973	0.38447
1974	0.38763
1975	0.39577
1976	0.40783
1977	0.3999
1978	0.44501
1979	0.45769
1980	0.46013
1981	0.46752
1982	0.47575
1983	0.46213
1984	0.46289
1985	0.47161
1986	0.48726
1987	0.48784
1988	0.498
1989	0.49448
1990	0.4971
2006	0.78363
2007	0.80447
2008	0.81021
2009	0.82903
2010	0.82029
2011	0.81592
2012	0.8159
2013	0.81298
2014	0.82836
2015	0.83113
2016	0.83816