Year	Percent
1970	42.2719
1971	40.54284
1972	41.54842
1973	49.53316
1974	54.31638
1975	56.46493
1976	57.55191
1977	57.25767
1978	56.0974
1979	55.32473
1980	55.44185
1981	58.47426
1982	59.70753
1983	61.73067
1984	61.53803
1985	62.05641
1986	63.26741
1987	62.50435
1988	64.71349
1989	61.95034
1990	62.6718
1991	60.6534
1992	60.44621
1993	60.71859
1994	61.79727
1995	63.42972
1996	61.08358
1997	61.94102
1998	63.60079
1999	60.53143
2000	61.58388
2001	59.10231
2002	62.20999
2003	60.33284
2004	63.03499
2005	66.10989
2006	66.64505
2007	73.33051
2009	71.50991
2010	75.51937
2011	75.4354
2012	75.8805
2013	77.10456
2014	75.99549
2015	80.58824