Year	Percent
1980	30.25451
1981	29.02468
1982	28.71988
1983	28.96096
1984	30.09512
1985	30.1135
1986	29.7071
1987	29.38439
1988	29.19119
1989	28.96832
1990	27.65494
1991	25.93247
1992	24.94914
1993	24.51609
1994	25.51035
1995	27.37478
1996	26.63185
1997	26.80654
1998	26.88529
1999	26.44225
2000	26.83958
2001	26.42702
2002	26.03935
2003	25.78293
2004	26.21475
2005	26.12635
2006	26.5803
2007	26.73945
2008	25.85185
2009	23.63618
2010	25.43607
2011	24.89052
2012	23.74427
2013	23.06133
2014	22.80156
2015	21.68187
2016	21.69705
2017	22.14192