Year	Percent
1970	81.22677
1971	81.60291
1972	82.14286
1973	82.662
1974	83.16151
1975	83.50168
1976	84.13223
1977	84.44084
1978	84.89666
1979	85.33541
1980	85.91118
1981	86.31579
1982	86.85377
1983	87.24638
1984	87.74929
1985	88.11189
1986	88.46154
1987	89.05405
1988	89.50863
1989	89.94778
1990	90.37227
1991	90.78283
1992	91.19107
1993	91.69719
1994	92.06731
1995	92.43499
1996	92.90698
1997	93.36384
1998	93.80631
1999	94.23503
2000	94.65066
2001	95.05376
2002	95.44974
2003	95.82899
2004	96.3001
2005	96.5587
2006	97.00599
2007	97.34513
2008	97.57752
2009	97.99235
2010	98.30349
2011	98.69767
2012	98.9899
2013	99.27536
2014	99.55277
2015	99.82332