Year	Percent
1960	6.10647
1961	5.5634
1962	5.57794
1963	5.58757
1964	5.59225
1965	5.60149
1966	5.62458
1967	5.66679
1968	5.66967
1969	5.58202
1970	5.50984
1971	3.92451
1972	3.8838
1973	3.98811
1974	4.28783
1975	4.68646
1976	5.1243
1977	5.47184
1978	5.73478
1979	5.85135
1980	5.89657
1981	5.91254
1982	5.96579
1983	5.98773
1984	5.98702
1985	5.9799
1986	6.17337
1987	6.49691
1988	6.58772
1989	6.84474
1990	7.17125
1991	7.5236
1992	7.7245
1993	7.64684
1994	7.27496
1995	6.46262
1996	5.68253
1997	5.23391
1998	4.93081
1999	4.80452
2000	4.79781
2001	4.80299
2002	4.78812
2003	4.7653
2004	4.73571
2005	4.69261
2006	4.66405
2007	4.63945
2008	4.61494
2009	4.58964
2010	4.56159
2011	4.53091
2012	4.49479
2013	4.44699
2014	4.37557
2015	4.2957
2016	4.21011
2017	4.12857