Year	Percent
1987	7.98
1988	8.42
1989	9.43
1990	9.54
1991	9.08
1992	7.96
1993	30.49
1994	31.74
1995	28
1996	26.38
1997	24.57
1998	23.82
1999	22.6
2000	20.81
2001	26.32
2002	25.18
2003	24.58
2004	24.56
2005	24.25
2006	24.23
2007	23.93
2008	23.43
2009	25.71
2010	25.49
2011	25.56
2012	25.72
2013	24.81
2014	26.14
2015	24.69
2016	24.24
2017	24.3