Year	Percent
1960	4.52922
1961	4.59989
1962	4.62823
1963	7.12975
1964	7.12542
1965	7.08413
1966	7.05162
1967	8.24223
1968	8.20432
1969	8.1534
1970	8.11694
1971	8.06631
1972	8.0328
1973	7.99356
1974	7.98392
1975	7.97726
1976	7.99151
1977	7.96677
1978	5.8347
1979	4.48148
1980	4.46091
1981	4.45398
1982	4.44635
1983	4.44862
1984	4.43821
1985	4.42171
1986	4.41193
1987	4.41617
1988	4.34069
1989	3.51317
1990	3.60602
1991	3.72036
1992	3.80346
1993	3.88987
1994	3.9562
1995	4.0169
1996	4.06994
1997	4.11721
1998	4.151
1999	4.15625
2000	4.14794
2001	4.08035
2002	3.58988
2003	3.60261
2004	3.62259
2005	3.64451
2006	3.6775
2007	3.69985
2008	3.71708
2009	3.74733
2010	3.77032
2011	3.78601
2012	3.80956
2013	3.81856
2014	3.91055
2015	4.00823
2016	4.09039
2017	4.18472