Year	Number of
1990	2.19
1991	2.24
1992	2.28
1993	2.31
1994	2.35
1995	2.39
1996	2.5
1997	2.62
1998	2.74
1999	2.86
2000	2.99
2001	3.08
2002	3.19
2003	3.3
2004	3.43
2005	3.57
2006	3.7
2007	3.84
2008	4
2009	4.16
2010	4.33
2011	4.52
2012	4.72
2013	4.93
2014	5.16
2015	5.4
2016	5.64