Year	Percent
1961	18.00326
1962	18.2319
1963	18.46054
1964	18.69187
1965	18.92185
1966	19.14914
1967	19.44503
1968	19.74091
1969	19.96955
1970	20.30847
1971	20.64471
1972	20.98094
1973	21.30103
1974	21.85515
1975	22.25863
1976	23.13283
1977	22.99834
1978	22.86384
1979	22.72935
1980	22.6621
1981	22.52761
1982	22.32587
1983	22.25863
1984	22.19138
1985	22.15372
1986	22.1053
1987	21.73814
1988	21.72603
1989	21.74621
1990	21.38307
1991	21.23513
1992	20.92176
1993	20.81551
1994	20.7792
1995	20.61781
1996	20.486
1997	20.47256
1998	20.46852
1999	20.25468
2000	20.32192
2001	20.37572
2002	20.93387
2003	20.74558
2004	21.43149
2005	21.36425
2006	21.24993
2007	21.15847
2008	20.66552
2009	20.17257
2010	19.67827
2011	19.18532
2012	18.69236
2013	18.19941
2014	17.70646
2015	17.21216
2016	16.71921
2017	16.22626
2018	15.7333
2019	15.23901
2020	14.74605
2021	14.25028