Year	Percent
1960	20.98707
1961	20.34751
1962	19.37995
1963	19.81054
1964	20.23739
1965	20.12538
1966	20.65282
1967	20.081
1968	20.73292
1969	23.14072
1970	25.07787
1971	25.39475
1972	26.26518
1973	28.13835
1974	34.73177
1975	29.66653
1976	32.48714
1977	33.0912
1978	31.80511
1979	33.83011
1980	35.77644
1981	36.9387
1982	36.31665
1983	35.87445
1984	38.05053
1985	37.95162
1986	32.96936
1987	32.84808
1988	34.02316
1989	36.32254
1990	35.53666
1991	35.36533
1992	33.9294
1993	33.18452
1994	35.62254
1995	36.88433
1996	37.37176
1997	40.41414
1998	41.80682
1999	42.96222
2000	48.93016
2001	47.36677
2002	44.23391
2003	42.97135
2004	43.62773
2005	44.05723
2006	44.75934
2007	44.80158
2008	45.67718
2009	38.86869
2010	42.9438
2011	46.00886
2012	46.3191
2013	44.90911
2014	44.16717
2015	44.28233
2016	43.56751
2017	44.91391