Year	Percent
1973	6.13983
1974	10.04152
1975	8.64452
1976	19.73211
1977	24.26648
1978	21.30263
1979	19.09838
1980	20.9409
1981	20.44274
1982	26.21261
1983	29.50006
1984	28.48833
1985	28.8235
1986	35.6444
1987	39.1603
1988	37.80086
1989	36.37517
1990	38.13288
1991	41.10773
1992	41.34397
1993	41.46035
1994	44.48178
1995	40.24565
1996	31.83461
1997	28.73778
1998	30.16561
1999	31.13473
2000	28.31641
2001	26.92982
2002	29.32795
2003	29.4691
2004	29.0271
2005	25.51767
2006	26.57517
2007	25.46627
2008	23.71971
2009	22.99902
2010	21.57066
2011	19.64852
2012	19.74464
2013	20.97739
2014	19.33892
2015	18.62328
2016	17.56241