Year	Percent
1960-01-01	13.500
1961-01-01	12.595
1962-01-01	12.870
1963-01-01	14.270
1964-01-01	14.600
1965-01-01	14.825
1966-01-01	15.995
1967-01-01	15.705
1968-01-01	15.635
1969-01-01	15.230
1970-01-01	16.285
1971-01-01	16.510
1972-01-01	18.260
1973-01-01	18.940
1974-01-01	18.960
1975-01-01	20.400
1976-01-01	21.670
1977-01-01	22.745
1978-01-01	23.210
1979-01-01	23.600
1980-01-01	24.020
1981-01-01	25.150
1982-01-01	25.460
1983-01-01	27.505
1984-01-01	28.830
1985-01-01	27.250
1986-01-01	27.585
1987-01-01	28.490
1988-01-01	27.810
1989-01-01	29.090
1990-01-01	28.340
1991-01-01	29.505
1992-01-01	28.320
1993-01-01	28.555
1994-01-01	26.360
1995-01-01	26.225
1996-01-01	28.070
1997-01-01	28.860
1998-01-01	30.060
1999-01-01	31.550
2000-01-01	32.560
2001-01-01	34.120
2002-01-01	33.500
2003-01-01	35.390
2004-01-01	35.715
2005-01-01	35.560
2006-01-01	35.490
2007-01-01	37.430
2008-01-01	38.660
2009-01-01	42.680
2010-01-01	41.770
2011-01-01	42.040
2012-01-01	42.970
2013-01-01	44.590
2014-01-01	47.660
2015-01-01	49.540