Year	Percent
1960	6.90132
1961	8.6015
1962	10.11913
1963	10.78006
1964	11.38363
1965	13.14597
1966	13.55625
1967	11.88361
1968	12.60041
1969	13.12857
1970	13.66119
1971	12.85294
1972	13.8116
1973	14.75349
1974	18.47065
1975	16.67386
1976	17.4726
1977	15.87719
1978	13.81561
1979	14.10115
1980	17.23231
1981	19.05527
1982	19.50886
1983	20.64827
1984	20.19442
1985	19.95845
1986	17.11277
1987	18.55473
1988	19.36854
1989	20.73667
1990	19.84275
1991	19.675
1992	19.74731
1993	19.19922
1994	21.28701
1995	22.89844
1996	23.21001
1997	25.53851
1998	26.68101
1999	28.34082
2000	31.62166
2001	30.21194
2002	28.50573
2003	27.66406
2004	29.03357
2005	29.66732
2006	30.7873
2007	31.69872
2008	30.44144
2009	23.82378
2010	26.82428
2011	29.16599
2012	29.23116
2013	28.96208
2014	30.28348
2015	30.67033
2016	29.92941
2017	31.42605