Year	Percent
1960	2.9017
1961	2.93859
1962	2.9753
1963	3.01127
1964	3.04667
1965	3.08119
1966	3.11735
1967	3.1534
1968	3.18491
1969	3.21112
1970	3.23499
1971	3.2512
1972	3.27079
1973	3.30969
1974	3.37194
1975	3.44445
1976	3.51353
1977	3.56277
1978	3.5852
1979	3.57605
1980	3.54417
1981	3.5203
1982	3.4978
1983	3.44548
1984	3.35945
1985	3.26077
1986	3.12878
1987	3.02531
1988	3.03359
1989	3.17846
1990	3.39218
1991	3.63594
1992	3.79362
1993	3.7731
1994	3.53952
1995	3.18293
1996	2.79509
1997	2.48739
1998	2.30722
1999	2.29432
2000	2.38972
2001	2.51653
2002	2.59464
2003	2.60317
2004	2.5173
2005	2.37521
2006	2.2239
2007	2.11455
2008	2.0632
2009	2.08734
2010	2.1617
2011	2.24686
2012	2.31294
2013	2.36069
2014	2.38118
2015	2.38247
2016	2.38044
2017	2.38303