Year	number
2005	18.50877192982456
2006	18.275862068965516
2007	18.24137931034483
2008	18.275862068965516
2009	18.03448275862069
2010	17.810344827586206
2011	17.32758620689655
2012	17.25862068965517
2013	17.0284375
2014	17.018769230769227
2015	16.800312499999997
2016	16.543384615384614
2017	16.514153846153846
2018	15.954
2019	15.478923076923078