Year	number
2004	6.627906976744186
2005	6.588235294117647
2006	6.627450980392157
2007	6.5
2008	6.480769230769231
2009	6.423076923076923
2010	6.377358490566038
2011	6.415094339622642
2012	6.425925925925926
2013	6.451864406779661
2014	6.417966101694915
2015	6.3162711864406775
2016	6.316271186440678
2017	6.316271186440678
2018	6.3272881355932205
2019	6.3213559322033905