Year	Percent
1971	18.67534
1972	19.01494
1978	23.57087
1979	24.59216
1980	23.05537
1981	23.80565
1982	23.69977
1983	25.02617
1984	26.12075
1985	25.18453
1986	27.5944
1988	31.65271
1989	32.16443
1990	32.92704
1992	31.16045
1994	30.17836
1996	31.6895
1997	32.43621
1998	35.15771
2000	32.63883
2001	36.99939
2002	40.50232
2003	41.24678
2004	42.94329
2005	47.66402
2006	46.53401
2007	48.36739
2008	56.31484
2009	57.88479
2010	57.92667
2011	61.7322
2012	62.04278
2013	62.92397
2014	64.24981
2015	62.54287
2016	63.49427