Year	Percent
1960	6.00463
1961	6.5977
1962	6.98593
1963	7.38048
1964	7.74605
1965	8.0537
1966	8.29147
1967	8.47415
1968	8.63854
1969	8.82876
1970	9.05923
1971	9.29659
1972	9.58568
1973	9.9161
1974	10.2756
1975	10.65114
1976	11.04597
1977	11.45866
1978	11.87571
1979	12.27468
1980	12.64184
1981	12.96742
1982	13.28238
1983	13.60587
1984	13.94465
1985	14.3138
1986	14.71918
1987	15.15679
1988	15.63404
1989	15.97234
1990	16.34893
1991	16.76879
1992	17.22952
1993	17.70741
1994	18.17791
1995	18.62044
1996	19.02778
1997	19.40494
1998	19.77432
1999	20.16071
2000	20.58345
2001	21.04336
2002	21.53848
2003	22.05994
2004	22.59747
2005	23.1381
2006	23.02199
2007	22.90315
2008	22.78393
2009	22.66483
2010	22.54987
2011	22.43832
2012	22.33097
2013	22.22784
2014	22.1332
2015	22.04786
2016	21.97209
2017	21.90527