Year	Percent
1960	2.23358
1961	2.31998
1962	2.39788
1963	2.45458
1964	2.48416
1965	2.49596
1966	2.47866
1967	2.46469
1968	2.49836
1969	2.59384
1970	2.71437
1971	2.87958
1972	2.98913
1973	2.91901
1974	2.6294
1975	2.2203
1976	1.76549
1977	1.42433
1978	1.31756
1979	1.52034
1980	1.93169
1981	2.39666
1982	2.7755
1983	3.04716
1984	3.16953
1985	3.1914
1986	3.18495
1987	3.20665
1988	3.25451
1989	3.34167
1990	3.44335
1991	3.54364
1992	3.60087
1993	3.58505
1994	3.48438
1995	3.33318
1996	3.16906
1997	3.03241
1998	2.93742
1999	2.89723
2000	2.89289
2001	2.89575
2002	2.88522
2003	2.86362
2004	2.82586
2005	2.77872
2006	2.73013
2007	2.68982
2008	2.66115
2009	2.64707
2010	2.64159
2011	2.63767
2012	2.6276
2013	2.60893
2014	2.57907
2015	2.54147
2016	2.50182
2017	2.4637