Year	Number
1970	8.60610965
1971	8.6149302043
1972	8.6237507586
1973	8.632571313
1974	8.6413918673
1975	8.6502124216
1976	8.955371232
1977	9.2605300425
1978	9.5656888529
1979	9.8708476634
1980	10.1760064738
1981	10.3605098119
1982	10.5450131499
1983	10.729516488
1984	10.914019826
1985	11.0985231641
1986	12.0558365365
1987	13.013149909
1988	13.9704632814
1989	14.9277766539
1990	15.8850900263
1991	16.0732348776
1992	16.2613797289
1993	16.0921167982
1994	15.9228538674
1995	15.7535909367
1996	16.6619461865
1997	17.5703014364
1998	17.7932429699
1999	18.0161845033
2000	18.2391260368
2001	18.4620675703
2002	18.6850091038
2003	18.9598084834
2004	19.234607863
2005	19.5094072426
2006	19.4853327938
2007	19.4612583451
2008	19.6202205139
2009	19.780194214
2010	19.9381448513
2011	20.0991300829
2012	20.0991300829
2013	20.0788994538
2014	20.0606918875
2015	20.0445073842
2016	20.5057657293
2017	20.5017196035
2018	20.4774428485
2019	20.4632814081
2020	20.4774428485