Year	Number of
1990	2.94
1991	2.98
1992	3.01
1993	3.05
1994	3.1
1995	3.15
1996	3.12
1997	3.09
1998	3.07
1999	3.06
2000	3.07
2001	3.01
2002	2.97
2003	2.93
2004	2.9
2005	2.88
2006	2.89
2007	2.9
2008	2.93
2009	2.96
2010	3.01
2011	3.16
2012	3.31
2013	3.5
2014	3.69
2015	3.9
2016	4.13