Year	Number
1970	8.9785600599
1971	9.8792247917
1972	10.7798895235
1973	11.6805542552
1974	12.581218987
1975	13.4818837188
1976	14.3057766127
1977	15.1296695066
1978	15.9535624005
1979	16.7774552944
1980	17.6013481884
1981	17.9196704428
1982	18.2379926973
1983	18.5563149518
1984	18.8746372063
1985	19.1929594607
1986	19.1761071061
1987	19.1592547514
1988	19.1424023968
1989	19.1255500421
1990	19.1086976875
1991	18.9620197859
1992	18.8153418843
1993	18.6686639828
1994	16.8242673907
1995	14.9798707986
1996	13.3227225915
1997	11.6655743844
1998	11.9464469619
1999	11.8434603502
2000	9.0347345754
2001	8.4804793559
2002	8.6340230316
2003	7.9655462972
2004	6.6697874731
2005	5.7204381612
2006	4.9901694598
2007	6.4450894111
2008	6.7596666979
2009	6.4375994757
2010	5.822488531
2011	6.171706769
2012	6.0687201573
2013	6.0088006741
2014	5.8814717723
2015	6.0528040446
2016	5.9676060294
2017	6.3411665574
2018	6.0069281902
2019	6.0069281902
2020	6.0069281902