Year	Number of Years
1960-01-01	41.242
1961-01-01	41.366
1962-01-01	41.526
1963-01-01	41.710
1964-01-01	41.893
1965-01-01	42.083
1966-01-01	42.298
1967-01-01	42.486
1968-01-01	42.550
1969-01-01	42.369
1970-01-01	41.566
1971-01-01	39.699
1972-01-01	36.676
1973-01-01	32.667
1974-01-01	28.040
1975-01-01	23.595
1976-01-01	20.317
1977-01-01	18.907
1978-01-01	19.725
1979-01-01	22.744
1980-01-01	27.536
1981-01-01	33.342
1982-01-01	39.157
1983-01-01	44.173
1984-01-01	48.025
1985-01-01	50.563
1986-01-01	51.916
1987-01-01	52.587
1988-01-01	52.999
1989-01-01	53.302
1990-01-01	53.595
1991-01-01	53.916
1992-01-01	54.228
1993-01-01	54.514
1994-01-01	54.820
1995-01-01	55.189
1996-01-01	55.653
1997-01-01	56.212
1998-01-01	56.862
1999-01-01	57.604
2000-01-01	58.432
2001-01-01	59.335
2002-01-01	60.283
2003-01-01	61.241
2004-01-01	62.186
2005-01-01	63.088
2006-01-01	63.927
2007-01-01	64.695
2008-01-01	65.391
2009-01-01	66.010
2010-01-01	66.555
2011-01-01	67.034
2012-01-01	67.466
2013-01-01	67.868
2014-01-01	68.251
2015-01-01	68.620
2016-01-01	68.981