Year	Percent
1997	19.16
1998	16.9258
1999	15.4646
2000	15.9314
2001	17.1401
2002	21.9642
2003	20.0035
2004	18.6812
2005	18.4268
2006	20.1882
2007	18.7257
2008	18.2603
2009	20.0624
2010	22.1139
2011	23.7136
2012	23.0438
2013	24.581
2014	24.6954
2015	23.3703
2016	21.8221
2017	18.6285