Year	Number
1970	21.28077
1971	21.82807
1972	21.95154
1973	22.54937
1974	23.45488
1975	24.20426
1976	24.70324
1977	25.39657
1978	25.74172
1979	26.0272
1980	26.64102
1981	27.08108
1982	25.4269
1983	25.94244
1984	25.56915
1985	26.78162
1986	25.20462
1987	24.21373
1988	23.02462
1989	22.27415
1990	22.47455
1991	22.46225
1992	22.42808
1993	22.41334
1994	22.85461
1995	23.78258
1996	23.77591
1997	23.88319
1998	24.1289
1999	24.22114
2000	24.19368
2001	24.10106
2002	24.03936
2003	23.95412
2004	25.07748
2005	24.76394
2006	23.65815
2007	22.71599
2008	22.14932
2009	21.95194
2010	22.2158
2011	20.09817
2012	22.22636
2013	20.68247
2014	20.17024
2015	19.77498
2016	20.42466
2017	20.29485
2018	20.52323