Year	Percent
1970	28.19217
1971	27.39029
1972	27.65278
1973	28.2253
1974	29.11681
1975	24.89401
1976	26.45922
1977	25.58416
1978	24.31335
1979	24.85749
1980	25.74806
1981	23.51853
1982	23.79789
1983	21.75041
1984	21.22923
1985	21.16858
1986	21.75692
1987	22.26294
1988	23.34268
1989	24.26837
1990	24.40966
1991	23.60216
1992	21.96001
1993	19.54341
1994	20.31737
1995	20.51314
1996	19.62227
1997	19.452
1998	20.68051
1999	21.3639
2000	22.48805
2001	22.16487
2002	21.31558
2003	21.18613
2004	21.8896
2005	22.45262
2006	23.23771
2007	24.16481
2008	24.12988
2009	21.33316
2010	21.94629
2011	23.22091
2012	22.62663
2013	22.28719
2014	22.70983
2015	22.71232
2016	22.68578
2017	23.4593