Year	Percent
1960	25.14018
1961	25.56182
1962	25.99716
1963	26.4459
1964	26.91123
1965	27.38853
1966	27.8873
1967	28.40562
1968	28.93773
1969	29.46741
1970	29.89464
1971	30.17405
1972	30.45251
1973	30.74151
1974	31.067
1975	31.44171
1976	31.87152
1977	32.34855
1978	32.87794
1979	33.45566
1980	34.04675
1981	34.23083
1982	34.45505
1983	34.71552
1984	35.01068
1985	35.33371
1986	35.69061
1987	36.07952
1988	36.49544
1989	36.92534
1990	37.26324
1991	37.35099
1992	37.43874
1993	37.53507
1994	37.65918
1995	37.82073
1996	38.02069
1997	38.25187
1998	38.51988
1999	38.82533
2000	39.04234
2001	39.08592
2002	39.16047
2003	39.24392
2004	39.30972
2005	39.33766
2006	39.3238
2007	39.27633
2008	39.21253
2009	39.15269
2010	39.11697
2011	39.12703
2012	39.15836
2013	39.20716
2014	39.27264
2015	39.35178
2016	39.31354
2017	39.29156