Year	Percent
1973	25.98
1974	25.74
1975	26.06
1976	25.45
1977	23.55
1978	21.27
1979	21.1
1982	13.45
1983	21.76
1984	20.84
1985	20.09
1986	19.3
1987	17.61
1988	15.01
1989	17.42
1990	19.32
1992	18.29
1993	18.66
1994	19.04
1995	21.18
1996	23.55
1997	22.2
1998	25.02
1999	14.48
2004	22.95
2013	22.36
2015	22.9