Year	Percent
1970	40.81081
1971	41.36126
1972	41.87817
1973	42.50614
1974	43.09524
1975	43.87991
1976	44.51902
1977	45.2381
1978	45.91195
1979	46.65314
1980	47.34774
1981	48.09886
1982	48.98711
1983	49.91087
1984	50.68966
1985	51.75292
1986	52.82714
1987	53.83412
1988	55
1989	56.15836
1990	57.38636
1991	58.59697
1992	60
1993	61.31953
1994	62.86073
1995	64.39024
1996	65.66986
1997	66.86251
1998	68.05075
1999	69.161
2000	70.23411
2001	71.27193
2002	72.27616
2003	73.21998
2004	74.05858
2005	75.05155
2006	75.91463
2007	76.57658
2008	77.39388
2009	78.1889
2010	78.86647
2011	79.52607
2012	80.16838
2013	80.86876
2014	81.4781
2015	81.98198