Year	Percent
1960	2.51143
1961	2.48547
1962	2.47074
1963	2.49204
1964	2.56044
1965	2.6562
1966	2.75967
1967	2.84004
1968	2.87998
1969	2.8691
1970	2.82719
1971	2.77939
1972	2.74917
1973	2.74112
1974	2.76251
1975	2.80382
1976	2.83883
1977	2.86682
1978	2.90887
1979	2.96689
1980	3.02898
1981	3.08542
1982	3.11993
1983	3.12239
1984	3.08712
1985	3.02342
1986	2.94895
1987	2.86973
1988	2.77735
1989	2.67294
1990	2.55904
1991	2.44724
1992	2.33289
1993	2.202
1994	2.05175
1995	1.89312
1996	1.73926
1997	1.60354
1998	1.49081
1999	1.40713
2000	1.34966
2001	1.30145
2002	1.26723
2003	1.26858
2004	1.31092
2005	1.3834
2006	1.45971
2007	1.53404
2008	1.6213
2009	1.72072
2010	1.82136
2011	1.92479
2012	2.00661
2013	2.03609
2014	2.00067
2015	1.91996
2016	1.82546
2017	1.73846