Year	Percent
1960	3.02411
1961	2.36601
1962	1.78523
1963	1.43648
1964	1.39891
1965	1.58613
1966	1.80935
1967	1.97916
1968	2.17886
1969	2.38904
1970	2.60171
1971	2.84522
1972	3.06957
1973	3.17779
1974	3.14501
1975	3.03126
1976	2.90125
1977	2.83195
1978	2.86074
1979	3.00563
1980	3.23048
1981	3.42241
1982	3.58057
1983	3.78842
1984	4.05573
1985	4.32554
1986	4.60392
1987	4.78039
1988	4.72715
1989	4.4188
1990	3.96838
1991	3.50301
1992	3.13366
1993	2.88138
1994	2.78438
1995	2.79555
1996	2.81905
1997	2.8283
1998	2.86375
1999	2.92641
2000	3.00146
2001	3.08944
2002	3.16498
2003	3.21501
2004	3.2246
2005	3.21059
2006	3.19028
2007	3.17471
2008	3.16273
2009	3.15924
2010	3.15714
2011	3.1536
2012	3.14312
2013	3.12465
2014	3.09928
2015	3.06731
2016	3.03358
2017	2.99931