Year	Percent
1960	5.19715
1961	4.76112
1962	4.6932
1963	4.72433
1964	4.74842
1965	4.75788
1966	4.76024
1967	4.7642
1968	4.8013
1969	4.89012
1970	4.8537
1971	4.1917
1972	4.28533
1973	4.25964
1974	4.10598
1975	3.87208
1976	3.64261
1977	3.47933
1978	3.35686
1979	3.29507
1980	3.29598
1981	3.25641
1982	3.11897
1983	3.28392
1984	3.71167
1985	4.25457
1986	4.83086
1987	5.25349
1988	5.41302
1989	5.24646
1990	4.89571
1991	4.49138
1992	4.70362
1993	4.71448
1994	4.64773
1995	4.6636
1996	4.71512
1997	4.68905
1998	4.53717
1999	4.22167
2000	3.81946
2001	3.4006
2002	4.07016
2003	3.9896
2004	3.85578
2005	3.8091
2006	3.78644
2007	3.76245
2008	3.81935
2009	3.94934
2010	4.14563
2011	4.35836
2012	4.60374
2013	4.69996
2014	4.68684
2015	4.59202
2016	4.44505
2017	4.31094