Year	Percent
1960	2.37373
1961	2.40631
1962	2.43836
1963	2.46932
1964	2.49926
1965	2.52864
1966	2.55625
1967	2.58402
1968	2.61534
1969	2.65053
1970	2.68749
1971	2.72318
1972	2.75504
1973	2.78254
1974	2.80454
1975	2.82194
1976	2.83686
1977	2.84981
1978	2.86005
1979	2.86793
1980	2.87321
1981	2.88117
1982	2.88707
1983	2.8836
1984	2.86863
1985	2.84859
1986	2.82768
1987	2.81576
1988	2.82092
1989	2.84671
1990	2.8862
1991	2.92429
1992	2.95777
1993	2.996
1994	3.03872
1995	3.08108
1996	3.12164
1997	3.15182
1998	3.16206
1999	3.14888
2000	3.11971
2001	3.08586
2002	3.05349
2003	3.01997
2004	2.98668
2005	2.9539
2006	2.92115
2007	2.88822
2008	2.85561
2009	2.82353
2010	2.79265
2011	2.76181
2012	2.73375
2013	2.71305
2014	2.7011
2015	2.69458
2016	2.68887
2017	2.68058