Year	Percent
1960	2.88976
1961	2.90035
1962	2.91051
1963	3.04126
1964	3.09752
1965	3.10313
1966	3.11142
1967	3.11419
1968	3.09033
1969	3.03745
1970	2.96492
1971	2.88981
1972	2.83707
1973	3.75359
1974	3.78505
1975	3.82537
1976	3.86701
1977	3.89829
1978	3.94842
1979	4.00278
1980	4.06667
1981	4.11915
1982	4.16424
1983	4.58519
1984	4.57448
1985	4.52503
1986	4.47383
1987	4.41797
1988	4.3527
1989	4.26598
1990	4.18289
1991	4.0877
1992	3.9969
1993	3.7355
1994	3.63714
1995	3.57066
1996	3.50689
1997	3.43168
1998	3.35541
1999	3.26793
2000	3.17442
2001	3.08197
2002	2.99524
2003	2.45115
2004	2.26769
2005	2.16661
2006	2.06094
2007	1.96797
2008	1.90559
2009	1.87898
2010	1.87816
2011	1.8857
2012	1.88896
2013	1.8301
2014	1.7933
2015	1.76733
2016	1.74312
2017	1.71848