Year	Number of
1990	2.58
1991	2.68
1992	2.8
1993	2.91
1994	3.02
1995	3.13
1996	3.01
1997	3.09
1998	3.01
1999	2.99
2000	3.04
2001	3.03
2002	3.04
2003	3.07
2004	3.06
2005	3.13
2006	3.2
2007	3.24
2008	3.31
2009	3.35
2010	3.45
2011	3.6
2012	3.77
2013	3.94
2014	4.02
2015	4.13
2016	4.26