Year	Number
1970	5.8773448773
1971	5.8952380952
1972	5.9131313131
1973	5.931024531
1974	5.9489177489
1975	5.9668109668
1976	5.9584415584
1977	5.9500721501
1978	5.9417027417
1979	5.9333333333
1980	5.924963925
1981	5.5971139971
1982	5.2692640693
1983	4.9414141414
1984	4.6135642136
1985	4.2857142857
1986	4.2867965368
1987	4.2878787879
1988	4.288961039
1989	4.29004329
1990	4.2828282828
1991	4.2864357864
1992	4.29004329
1993	4.1707551708
1994	4.0514670515
1995	3.9321789322
1996	3.9220779221
1997	3.911976912
1998	3.9009139009
1999	3.8898508899
2000	3.8787878788
2001	3.8614718615
2002	3.8614718615
2003	3.8398268398
2004	3.8181818182
2005	3.7965367965
2006	3.7958152958
2007	3.7950937951
2008	3.8234728235
2009	3.8518518519
2010	3.8802308802
2011	3.7896103896
2012	3.698989899
2013	3.6083694084
2014	3.5177489177
2015	3.4271284271
2016	3.4582972583
2017	3.4894660895
2018	3.5206349206
2019	3.5518037518
2020	3.582972583