Year	Percent
1972	17.78923
1973	18.95249
1974	17.86407
1975	20.22511
1976	19.94827
1977	21.26922
1978	22.61766
1979	21.79671
1980	20.99139
1981	21.63864
1982	22.7812
1983	23.06769
1984	22.97892
1985	21.99963
1986	22.5169
1987	20.56177
1988	20.50089
1989	20.15141
1990	19.9534
1991	20.38441
1992	20.49935
1993	21.00798
1994	20.76414
1995	20.48581
1996	19.83762
1997	18.44103
1998	19.18316
1999	19.13117
2000	19.29243
2001	19.59744
2002	19.54051
2003	18.74521
2004	18.67215
2005	18.8775
2006	18.51011
2007	19.07082
2008	20.50181
2009	20.04816
2010	20.13909
2011	21.19311
2012	21.12789