Year	Number of
1990	2.86
1991	3.02
1992	3.1
1993	3.15
1994	3.23
1995	3.3
1996	3.31
1997	3.32
1998	3.26
1999	3.36
2000	3.26
2001	3.29
2002	3.43
2003	3.53
2004	3.52
2005	3.55
2006	3.7
2007	3.82
2008	3.94
2009	4.07
2010	4.22
2011	4.44
2012	4.53
2013	4.88
2014	5.06
2015	4.96
2016	5.03