Year	Percent
1972	0.21302
1973	0.16995
1974	0.17848
1975	0.18627
1976	0.25096
1977	0.2764
1978	0.2958
1979	0.3387
1980	0.3573
1981	0.35989
1982	0.37138
1983	0.38233
1984	0.40608
1985	0.4149
1986	0.42323
1987	0.43356
1988	0.46174
1989	0.51746
1990	0.55019
1991	0.58064
1992	0.61095
1993	0.62621
1994	0.65175
1995	0.66974
1996	0.69236
1999	0.72322
2000	0.74022
2001	0.77247
2002	0.78827
2003	0.81426
2005	0.8695
2006	0.91009
2007	0.93947
2008	0.95637
2009	0.96514
2010	1.01062
2011	1.03228
2012	1.05121
2013	1.06014
2014	1.07068
2015	1.07785
2016	1.07842
2017	1.08609