Year	Percent
1960	1.66875
1961	1.69407
1962	1.72065
1963	1.74602
1964	1.77041
1965	1.79328
1966	1.84245
1967	1.89629
1968	1.90654
1969	1.86172
1970	1.79507
1971	1.70872
1972	1.67996
1973	1.79665
1974	2.09509
1975	2.50714
1976	2.94529
1977	3.31348
1978	3.57792
1979	3.70901
1980	3.75084
1981	3.78818
1982	3.84501
1983	3.87698
1984	3.88649
1985	3.89001
1986	3.83336
1987	3.79554
1988	3.90195
1989	4.18017
1990	4.52804
1991	4.89263
1992	5.11108
1993	5.06022
1994	4.70619
1995	4.1852
1996	3.64535
1997	3.21719
1998	2.92622
1999	2.81213
2000	2.8177
2001	2.84267
2002	2.83618
2003	2.82921
2004	2.8117
2005	2.78766
2006	2.76759
2007	2.75497
2008	2.74247
2009	2.72913
2010	2.71303
2011	2.69737
2012	2.67605
2013	2.63989
2014	2.58591
2015	2.52025
2016	2.4515
2017	2.38647