Year	Percent
1960	2.93806
1961	4.92189
1962	6.5126
1963	6.19549
1964	6.55883
1965	7.28527
1966	8.21339
1967	8.86108
1968	9.19403
1969	8.98746
1970	8.5246
1971	4.60969
1972	3.19012
1973	3.05466
1974	3.22206
1975	3.52403
1976	3.86893
1977	4.09295
1978	4.12205
1979	3.9295
1980	3.57352
1981	3.19767
1982	4.61696
1983	4.62172
1984	4.53439
1985	4.56871
1986	4.60911
1987	4.62128
1988	4.64475
1989	4.64317
1990	4.64855
1991	4.6765
1992	4.28931
1993	4.21871
1994	4.11731
1995	3.97074
1996	3.81125
1997	3.67282
1998	3.62566
1999	3.67568
2000	3.79008
2001	3.92082
2002	4.42349
2003	4.53216
2004	4.49694
2005	4.41749
2006	4.32838
2007	4.26445
2008	4.17799
2009	4.11687
2010	4.04748
2011	3.97301
2012	3.9067
2013	3.79586
2014	3.69405
2015	3.59524
2016	3.50353
2017	3.41009