Year	Percent
1971	16.38
1972	18.94
1973	19.58
1974	25.57
1975	21.25
1976	19.26
1977	20.53
1978	20.32
1979	21.6
1980	18.91
1981	22.94
1982	24.66
1983	23.82
1984	22.55
1986	22.59
1987	23.79
1988	22.24
1989	21.37
1990	21.96
1991	24.2
1992	23.3
1993	25.66
1994	25.97
1995	28.25
1996	29.13
1997	29.92
1998	31.02
1999	33.09
2000	32.17
2001	35.11
2002	34.02
2003	35.32
2004	38.47
2005	38.9
2006	38.2
2007	38.48
2008	39.03
2009	40.2
2010	41.11
2011	39.53
2012	38
2013	39.24
2014	43.05
2015	44.04
2016	45.16