Year	Percent
1960	3.23741
1961	3.39563
1962	3.52364
1963	3.58672
1964	3.56854
1965	3.49692
1966	3.41865
1967	3.35173
1968	3.27618
1969	3.19584
1970	3.11451
1971	3.03569
1972	2.96428
1973	2.90647
1974	2.86446
1975	2.83673
1976	2.81207
1977	2.79501
1978	2.80122
1979	2.83429
1980	2.8832
1981	2.91119
1982	2.92827
1983	2.9742
1984	3.05418
1985	3.1398
1986	3.24132
1987	3.28776
1988	3.19292
1989	2.92986
1990	2.56734
1991	2.18993
1992	1.87579
1993	1.64259
1994	1.52067
1995	1.48335
1996	1.46078
1997	1.4383
1998	1.46221
1999	1.5358
2000	1.64335
2001	1.76864
2002	1.88757
2003	1.98384
2004	2.04622
2005	2.08358
2006	2.11316
2007	2.14562
2008	2.17602
2009	2.20662
2010	2.23386
2011	2.25555
2012	2.26577
2013	2.26002
2014	2.23625
2015	2.19868
2016	2.15588
2017	2.11142