Year	Percent
1960	3.19988
1961	3.23958
1962	3.26993
1963	3.27954
1964	3.26372
1965	3.2341
1966	3.19587
1967	3.16898
1968	3.17025
1969	3.20652
1970	3.26198
1971	3.33611
1972	3.39296
1973	3.39501
1974	3.32962
1975	3.23095
1976	3.1061
1977	3.02271
1978	3.0489
1979	3.20921
1980	3.44181
1981	3.66944
1982	3.82264
1983	3.88679
1984	3.84451
1985	3.72564
1986	3.59945
1987	3.47147
1988	3.29192
1989	3.05802
1990	2.79395
1991	2.52053
1992	2.27053
1993	2.06494
1994	1.91801
1995	1.81308
1996	1.73321
1997	1.65091
1998	1.55299
1999	1.42975
2000	1.29878
2001	1.1706
2002	1.08065
2003	1.06128
2004	1.13032
2005	1.26391
2006	1.41372
2007	1.55473
2008	1.70011
2009	1.8425
2010	1.97676
2011	2.10967
2012	2.22831
2013	2.30937
2014	2.3448
2015	2.34564
2016	2.33607
2017	2.32286