Year	Percent
1960	16.8899
1961	17.13807
1962	15.42866
1963	13.979
1964	15.47239
1965	16.66667
1966	16.28035
1967	17.46706
1968	17.66118
1969	18.04358
1970	17.97227
1971	18.28731
1972	18.68057
1973	18.64397
1974	18.32281
1975	17.88669
1976	16.35732
1977	15.86698
1978	17.43252
1979	17.3204
1980	17.77778
1981	17.38906
1982	17.30958
1983	16.90869
1984	16.42915
1985	16.74073
1986	16.18046
1987	15.92976
1988	16.75328
1989	16.36991
1990	16.32579
1991	17.00351
1992	15.84607
1993	14.36702
1994	14.55206
1995	13.55652
1996	16.39205
1997	15.92638
1998	15.54216
1999	14.9153
2000	15.01586
2001	15.30962
2002	15.45356
2003	15.62315
2004	16.20508
2005	17.01263
2006	19.38411
2007	19.30217
2008	18.58277
2009	18.74914
2010	18.54947
2011	18.90626
2012	18.62811
2013	18.02416
2014	17.44177
2015	17.30081
2016	17.45527
2017	17.49797