Year	number
2004	6.4423076923076925
2005	6.46875
2006	6.507692307692308
2007	6.476923076923077
2008	6.415384615384616
2009	6.318181818181818
2010	6.268656716417911
2011	6.268656716417911
2012	6.2898550724637685
2013	6.337638888888889
2014	6.295972222222222
2015	6.250821917808219
2016	6.264520547945206
2017	6.257123287671233
2018	6.224931506849314
2019	6.206438356164384