Year	Number of
1990	2.98
1991	3.04
1992	3.08
1993	2.99
1994	2.97
1995	3.11
1996	3.06
1997	3.11
1998	3.13
1999	3.16
2000	3.22
2001	3.33
2002	3.42
2003	3.57
2004	3.6
2005	3.75
2006	3.87
2007	3.93
2008	3.96
2009	4.02
2010	4.18
2011	4.31
2012	4.49
2013	4.6
2014	4.65
2015	4.74
2016	4.83