Year	Number of
1990	2.09
1991	2.21
1992	2.27
1993	2.24
1994	2.26
1995	2.36
1996	2.3
1997	2.34
1998	2.28
1999	2.22
2000	2.19
2001	2.18
2002	2.19
2003	2.24
2004	2.22
2005	2.27
2006	2.4
2007	2.46
2008	2.53
2009	2.63
2010	2.77
2011	2.91
2012	3.04
2013	3.21
2014	3.27
2015	3.37
2016	3.6