Year	Percent
1980	16.96005
1981	17.35196
1982	15.80928
1983	17.27724
1984	17.60405
1985	17.9838
1986	17.81949
1987	18.70416
1988	19.30336
1989	20.75777
1990	30.06554
1991	30.32638
1992	32.38158
1993	22.52794
1994	30.55919
1995	28.29413
1996	28.93081
1997	26.20735
1998	26.3779
1999	23.37704
2000	21.74719
2001	19.16407
2002	19.05351
2003	18.526
2004	18.21808
2005	19.21108
2006	19.09228
2007	16.90383
2008	18.75821
2009	19.26121
2010	18.14871
2011	17.76439
2012	17.00466
2013	17.51543
2014	18.7276
2015	18.11511
2016	17.42679
2017	18.54946