Year	Number
1970	5.1474956822
1971	5.4878756477
1972	5.8282556131
1973	6.1686355786
1974	6.509015544
1975	6.8493955095
1976	7.1392746114
1977	7.4291537133
1978	7.7190328152
1979	8.0089119171
1980	8.298791019
1981	8.832642487
1982	9.3664939551
1983	9.9003454231
1984	10.4341968912
1985	10.9680483592
1986	11.1621021466
1987	11.3561559339
1988	11.5502097212
1989	11.7442635085
1990	11.9383172958
1991	12.1323710831
1992	12.3264248705
1993	11.8400690846
1994	11.3537132988
1995	10.867357513
1996	10.3810017271
1997	10.3088658607
1998	10.2367299942
1999	10.1645941278
2000	10.0924582614
2001	10.0203223949
2002	9.9481865285
2003	9.8537708693
2004	9.7593552101
2005	9.6649395509
2006	9.5705238918
2007	9.4761082326
2008	9.3816925734
2009	10.9533678756
2010	9.274611399
2011	9.02417962
2012	8.7236614853
2013	9.067357513
2014	6.7754749568
2015	6.9602763385
2016	6.8376511226
2017	8.1805181347
2018	8.4947521554
2019	7.7082469775
2020	8.0701208981