Year	Percent
1976	26.2
1978	27.38
1996	26
1997	25.53
1998	26.12
1999	25.3
2000	24.64
2001	23.63
2002	21.55
2003	22.21
2004	23.16
2005	22.58
2006	22.36
2007	22.47
2008	21.45
2009	18.21
2010	18.48
2011	18.17
2012	18.57
2013	17.09
2014	17.79
2015	18.37
2016	19.01
2017	19.43