Year	Number of
1990	3.64
1991	3.9
1992	4.1
1993	4.35
1994	4.5
1995	4.28
1996	4.15
1997	4.07
1998	3.98
1999	3.89
2000	3.78
2001	3.69
2002	3.65
2003	3.71
2004	3.99
2005	4.08
2006	4.16
2007	4.25
2008	4.38
2009	4.55
2010	4.64
2011	4.72
2012	4.81
2013	4.88
2014	4.97
2015	5.06
2016	5.08