Year	Percent
1960	37.864
1961	38.272
1962	38.624
1963	38.977
1964	39.331
1965	39.686
1966	40.042
1967	40.399
1968	40.758
1969	41.117
1970	41.477
1971	41.838
1972	42.2
1973	42.563
1974	42.926
1975	43.291
1976	43.656
1977	43.81
1978	43.826
1979	43.842
1980	43.858
1981	43.874
1982	43.89
1983	43.906
1984	43.922
1985	43.938
1986	43.954
1987	43.877
1988	43.744
1989	43.611
1990	43.478
1991	43.345
1992	43.212
1993	43.079
1994	42.946
1995	42.814
1996	42.681
1997	42.658
1998	42.704
1999	42.75
2000	42.797
2001	42.843
2002	42.889
2003	42.935
2004	42.981
2005	43.027
2006	43.073
2007	43.078
2008	43.058
2009	43.039
2010	43.019
2011	43
2012	42.946
2013	42.893
2014	42.839
2015	42.785
2016	42.732
2017	42.705