Year	Percent
1960	2.50266
1961	2.55957
1962	2.61385
1963	2.65984
1964	2.69805
1965	2.73083
1966	2.75447
1967	2.7795
1968	2.82098
1969	2.88086
1970	2.94933
1971	3.01893
1972	3.07147
1973	3.09056
1974	3.07013
1975	3.02309
1976	2.96814
1977	2.92296
1978	2.8971
1979	2.89698
1980	2.91198
1981	2.93176
1982	2.93942
1983	2.92772
1984	2.89266
1985	2.84287
1986	2.79344
1987	2.75417
1988	2.72803
1989	2.71699
1990	2.71845
1991	2.72094
1992	2.72448
1993	2.73566
1994	2.75534
1995	2.78038
1996	2.81668
1997	2.85033
1998	2.86109
1999	2.84235
2000	2.81006
2001	2.75424
2002	2.71842
2003	2.75975
2004	2.89611
2005	3.08077
2006	3.28368
2007	3.42501
2008	3.44064
2009	3.30475
2010	3.07836
2011	2.83267
2012	2.63807
2013	2.51549
2014	2.48822
2015	2.52471
2016	2.57236
2017	2.59829