Year	Percent
1971	50.85837
1972	49.88861
1973	51.76797
1974	53.54308
1975	53.90231
1976	52.61952
1977	53.67333
1978	54.55924
1979	53.16363
1980	54.56478
1981	54.67135
1982	54.23024
1983	54.8278
1984	53.63602
1985	57.05939
1986	58.78039
1987	58.37924
1988	59.42119
1989	58.25283
1990	60.30564
1991	60.05054
1992	60.27021
1993	59.73479
1994	59.74407
1995	60.43331
1996	61.04094
1997	61.77991
1998	63.26584
1999	62.70213
2000	61.81178
2001	62.11853
2002	63.20711
2003	63.905
2004	63.89123
2005	64.30785
2006	64.35112
2007	63.42657
2008	64.35393
2009	65.49825
2010	64.36043
2011	64.31831
2012	65.00975
2013	64.32247
2014	65.39643
2015	65.60557