Year	Number of
1990	2.49
1991	2.49
1992	2.49
1993	2.48
1994	2.55
1995	2.66
1996	2.71
1997	2.6
1998	2.57
1999	2.68
2000	2.83
2001	2.87
2002	2.96
2003	3.09
2004	3.19
2005	3.22
2006	3.3
2007	3.35
2008	3.49
2009	3.52
2010	3.65
2011	3.81
2012	4
2013	4.08
2014	4.24
2015	4.27
2016	4.37