Year	Percent
1960	17.45184
1961	17.83203
1962	18.27659
1963	18.72946
1964	19.19575
1965	19.67437
1966	20.17059
1967	20.6835
1968	21.21643
1969	21.76652
1970	22.33982
1971	22.93703
1972	23.5314
1973	23.77346
1974	24.01919
1975	24.26484
1976	24.51076
1977	24.75601
1978	25.00955
1979	25.27702
1980	25.56439
1981	25.86999
1982	25.93741
1983	26.00954
1984	26.09091
1985	26.17615
1986	26.26847
1987	26.36919
1988	26.48093
1989	26.60302
1990	26.74041
1991	26.89194
1992	27.05969
1993	27.2453
1994	27.30209
1995	27.38182
1996	27.49013
1997	27.62256
1998	27.77855
1999	27.95331
2000	28.14425
2001	28.34783
2002	28.56551
2003	28.79366
2004	29.02984
2005	29.26887
2006	29.51266
2007	29.75945
2008	29.9769
2009	30.17485
2010	30.36654
2011	30.54976
2012	30.72719
2013	30.90159
2014	31.0824
2015	31.27399
2016	31.47888
2017	31.69384